PROSPECTUS SUMMARY

This summary highlights information contained elsewhere in this prospectus or incorporated by reference herein and is qualified in its entirety by the more detailed information and financial statements included elsewhere or incorporated by reference in this prospectus. You should read this entire prospectus carefully, including “Risk Factors” as well as the information incorporated by reference in this prospectus, before making an investment decision. As used throughout this prospectus, the terms “Company,” “Ocean Rig UDW,” “we,” “our” and “us” refer to Ocean Rig UDW Inc. and its subsidiaries except where the context otherwise requires. “DryShips” and the “selling shareholder” refer to DryShips Inc. (NASDAQ: DRYS), a Marshall Islands corporation and our parent company. Unless otherwise indicated, all references to “dollars” and “$” in this prospectus are to U.S. Dollars and the financial statements are presented in U.S. dollars and have been prepared in accordance with generally accepted accounting principles in the United States (U.S. GAAP). References to “real” or “R$” are to the official currency of Brazil, the real.

Our Company

We are an international offshore drilling contractor providing oilfield services for offshore oil and gas exploration, development and production drilling and specializing in the ultra-deepwater and harsh-environment segment of the offshore drilling industry. We seek to utilize our high-specification drilling units to the maximum extent of their technical capability and we believe that we have earned a reputation for operating performance excellence.

We currently own and operate two modern, fifth generation ultra-deepwater semi-submersible offshore drilling rigs, the Leiv Eiriksson and the Eirik Raude, and four sixth generation, advanced capability ultra-deepwater drillships, the Ocean Rig Corcovado, the Ocean Rig Olympia, the Ocean Rig Poseidon and the Ocean Rig Mykonos, delivered in January 2011, March 2011, July 2011 and September 2011, respectively, by Samsung Heavy Industries Co. Ltd., or Samsung. The Ocean Rig Corcovado, the Ocean Rig Olympia, the Ocean Rig Poseidon and the Ocean Rig Mykonos are “sister-ships” constructed by Samsung to the same high-quality vessel design and specifications and are capable of drilling in water depths of 10,000 feet.

We have additional newbuilding contracts with Samsung for the construction of three seventh generation ultra-deepwater drillships, which we refer to as our seventh generation hulls. These three newbuilding drillships are currently scheduled for delivery in July 2013, October 2013 and November 2013, respectively. The design of our seventh generation hulls reflects additional enhancements that, with the purchase of additional equipment, will enable the drillships to drill in water depths of 12,000 feet. We currently have a team overseeing the construction of the three newbuilding drillships at Samsung to help ensure that those drillships are built on time, to our exact vessel specifications and on budget, as was the case for our operating drillships. As of December 31, 2011, we had made pre-delivery payments for these three newbuilding drillships amounting to $726.7 million in the aggregate, which we financed with operating cash. The remaining total construction payments for these drillships, excluding financing costs, amounted to approximately $1.3 billion in the aggregate. Construction-related expenses include equipment purchases commissioning, supervision and commissions to related parties, excluding financing costs and fair value adjustments.

We believe that the Ocean Rig Corcovado, the Ocean Rig Olympia, the Ocean Rig Poseidon and the Ocean Rig Mykonos, as well as our three seventh generation hulls, are among the most

technologically advanced drillships in the world. The S10000E design, used for our operating drillships, was originally introduced in 1998 and, including our four existing drillships, a total of 56 drillships have been ordered using this base design, which has been widely accepted by customers, of which 31 have been delivered, as of February 2012, including the Ocean Rig Corcovado, the Ocean Rig Olympia, the Ocean Rig Poseidon and the Ocean Rig Mykonos. Among other technological enhancements, our drillships are equipped with dual activity drilling technology, which involves two drilling systems using a single derrick that permits two drilling-related operations to take place simultaneously. We estimate this technology saves between 15% and 40% in drilling time, depending on the well parameters. Each of our drillships is capable of drilling 40,000 feet at water depths of 10,000 feet and our seventh generation hulls will have the capacity to drill 40,000 feet at water depths of 12,000 feet.

Our revenue, earnings before interest, taxes, depreciation and amortization, or EBITDA, and net income for the year ended December 31, 2011 were $699.6 million, $344.4 million and $95.3 million, respectively. We believe EBITDA provides useful information to investors because it is a basis upon which we measure our operations and efficiency. Please see “Selected Historical Consolidated Financial and Other Data” for a reconciliation of EBITDA to net income, the most directly comparable U.S. generally accepted accounting principles, or U.S. GAAP, financial measure.

Our Fleet

Set forth below is summary information concerning our offshore drilling units as of April 9, 2012.

Newbuilding Drillships and Options to Purchase Newbuilding Drillships

As of the date of this prospectus, we have options with Samsung for the construction of up to three additional ultra deepwater drillships, which would be “sister-ships” to our operating drillships and our seventh generation hulls, with certain upgrades to vessel design and specifications. We have entered into shipbuilding contracts for three seventh generation hulls with deliveries scheduled in July 2013, October 2013 and November 2013, respectively. We have made total payments of $726.7 million to the shipyard in connection with our three newbuilding drillships and payment of pre-delivery installments. The estimated total project cost per drillship, excluding financing costs, ranges between $668.0 million and $678.0 million, consisting of $608.0 million of construction costs, upgrade costs to the existing drillship specifications of between $10.0 million and $20.0 million and construction-related expenses of $50.0 million. These upgrades include a seven ram blowout preventer, or BOP, a dual mud system and, with the purchase of additional equipment, the capability to drill up to 12,000 feet water depth.

On April 2, 2012, we entered into an addendum to our option contract with Samsung to extend the deadline for exercising the remaining three options for the construction of up to three additional seventh generation ultra-deepwater drillships from April 2, 2012 to October 4, 2012, with the first vessel being delivered in the fourth quarter of 2014 and the second and third vessels being delivered on the earliest available delivery dates based on the production schedule, as determined by Samsung in its reasonable discretion. We estimate the total project cost per drillship for the remaining three optional drillships, excluding financing costs, to range between $668.0 million and $678.0 million, based on the construction expenses for our seventh generation hulls described above and assuming the drillships are built with the same specifications as the seventh generation hulls described above. These figures provide an estimate only, and the actual project cost per optional drillship will depend on various factors, including market conditions at the time we may exercise any of the options.

Employment of Our Fleet

The Leiv Eiriksson is currently employed under a contract with Borders & Southern plc, or Borders & Southern, for drilling operations offshore the Falkland Islands at a maximum operating dayrate of $530,000 and a $3.0 million mobilization fee which was payable upon commencement, as well as mobilization and demobilization fees, including fuel costs, of $15.4 million and $12.6 million, respectively. On May 19, 2011, Borders & Southern exercised its option to extend the contract from a two well program to drill an additional two wells, which it assigned to Falkland Oil and Gas Limited, or Falkland Oil and Gas. The estimated duration for the four-well contract, including mobilization/demobilization periods, is approximately 230 days, with the contract scheduled to expire in the third quarter of 2012. The Eirik Raude was originally scheduled to commence this contract with Borders & Southern; however on May 5, 2011, we terminated the contract for the Eirik Raude and entered into a new contract for the Leiv Eiriksson on the same terms as the original contract for the Eirik Raude, with the exception of the fees payable upon mobilization and demobilization and certain other terms specific to the Leiv Eiriksson, including off-hire dates, period surveys and technical specifications. Following the completion of the contract with Borders & Southern, the Leiv Eiriksson is scheduled to commence a contract with a consortium coordinated by Rig Management Norway for the drilling of 15 wells on the Norwegian Continental Shelf at a maximum dayrate of $545,000, plus a mobilization fee of $70.0 million plus fuel to cover mobilization costs and upgrades to operate in the Norwegian Continental Shelf. The contract has a minimum duration of 1,070 days and includes three options of up to 6 wells each that must be exercised prior to the expiry of the firm contract period.

In January 2012, following the completion of the contract with Tullow Oil plc, or the Tullow Oil contract, discussed below, the Eirik Raude commenced a contract with Anadarko Cote d’Ivoire Company, or Anadarko, at a maximum dayrate of $535,000 for the drilling of two wells offshore West Africa. The term of the contract with Anadarko is approximately 100 days and the operator may extend the duration of the contract for an additional period or additional well or wells. Prior to its commencement of the contract with Anadarko, the Eirik Raude was employed under the Tullow Oil contract at a maximum dayrate of $665,000 for the drilling of one well offshore West Africa, which was amended to extend the contract term until mid-January 2012. Following the completion of the contract with Anadarko, the Eirik Raude is scheduled to commence a contract with Ophir Services Pty Ltd., or Ophir Services, at a maximum dayrate of $647,651, which may be increased to $668,984 in the event we are required to recruit certain additional crew, for the drilling of three wells offshore Equatorial Guinea. In addition, we are entitled to a mobilization fee of $568,400 per day, plus the cost of fuel, and a demobilization fee of $4.75 million; however, we are not entitled to the demobilization fee in the event the Eirik Raude engages in drilling operations with other companies in direct continuation of the contract with Ophir Services. The contract has an estimated duration of 60 days into the third quarter of 2012 and Ophir Services has the option to extend the contract term by one additional well, which is estimated to extend the duration of the contract by an additional 20 days.

Following the completion of the contract with Ophir Services discussed above, the Eirik Raude is scheduled to commence a contract with European Hydrocarbons Limited, or European Hydrocarbons, for the drilling of two wells offshore West Africa at a maximum dayrate of $718,750, which includes the operating dayrate of $625,000 plus the maximum performance bonus available under the contract. The operating rate may be increased to $671,875 in the event we are unable to use certain crew that were on the rig under the contract immediately prior to our contract with European Hydrocarbons or we are required to recruit certain additional crew, in which case the maximum dayrate available under the contract, including the maximum performance bonus, would be $772,656. We are also entitled to a mobilization fee under the contract of $15.0 million plus the cost of fuel consumed during the mobilization period. The contract has an estimated duration of 84 days and the customer has the option to extend the contract for (i) such number of days for the testing of one additional well at the applicable operating dayrate or (ii) the drilling of one additional well, which is estimated to extend the duration of the contract by 40 days, for a fee of approximately $24.9 million. The contract is scheduled to commence in the third quarter of 2013.

The Ocean Rig Corcovado is currently employed under a three-year contract, plus a mobilization period, with Petróleo Brasileiro S.A., or Petrobras Brazil, for drilling operations offshore Brazil at a maximum dayrate of $452,000 (including service fees of $83,000 per day, based on the contracted rate of R$ 153,000 per day and the April 4, 2012 exchange rate of R$ 1.83:USD $1.00), plus a mobilization fee of $30.0 million. The Ocean Rig Corcovado is currently undergoing acceptance testing under the contract and is expected to commence drilling operations in April 2012. The contract is scheduled to be completed in the first quarter of 2015.

The Ocean Rig Olympia is currently operating under contracts to drill a total of five wells for exploration drilling offshore Ghana and the Ivory Coast at a maximum operating dayrate of $498,000 and a daily mobilization rate of $180,000, plus fuel costs. The original counterparty to the contracts was Vanco Cote d’Ivoire Ltd. and Vanco Ghana Ltd, or collectively, Vanco. On December 8, 2011, the remainder of the contracts was novated to Tullow Ghana. The Ocean Rig Olympia is currently performing drilling operations under the contract in Ghana. In February

2012, we agreed with Tullow Ghana to extend the number of operational days under the contracts from 108 to 120 and increase the dayrate to $600,000 per day from April 7, 2012. The contracts are scheduled to expire at the end of May 2012. We have received a letter of award from Total E&P Angola, or Total, for a three-year contract for drilling operations offshore West Africa that is scheduled to commence upon the completion of the contract with Tullow Ghana, at a maximum dayrate of $584,450, plus mobilization and demobilization fees of $9.0 million and $3.5 million, respectively, plus the cost of fuel. Under the contract, Total will have the option to extend the term of the contract for two additional one-year periods at the dayrate specified above, adjusted annually for inflation, with the first option exercisable within one year from the commencement date under the drilling contract, and the second option exercisable within one year after the date of exercise of the first option. The contract is subject to final documentation and customary conditions. The letter of award expires on April 30, 2012.

The Ocean Rig Poseidon commenced a contract with Petrobras Tanzania, a company related to Petrobras Oil & Gas B.V., or Petrobras Oil & Gas, on July 29, 2011 for drilling operations in Tanzania and West Africa for a period of 544 days, plus a mobilization period, at a maximum dayrate of $632,000, including a bonus of up to $46,000. In addition, we are entitled to receive a separate dayrate of $422,500 for up to 60 days during relocation and a mobilization dayrate of $317,000, plus the cost of fuel. The Ocean Rig Poseidon is currently performing drilling operations offshore Tanzania. The contract is scheduled to expire in April 2013.

The Ocean Rig Mykonos commenced a three-year contract, plus a mobilization period, with Petrobras Brazil, on September 30, 2011, for drilling operations offshore Brazil at a maximum dayrate of $447,000 (including service fees of $83,000 per day, based on the contracted rate of R$ 152,000 per day and the April 4, 2012 exchange rate of R$ 1.83: USD $1.00), plus a mobilization fee of $30.0 million. The Ocean Rig Mykonos is currently performing drilling operations under the contract. The contract is scheduled to expire in February 2015.

The total contracted backlog under our drilling contracts and the Total letter of award for the Ocean Rig Olympia as of March 31, 2012 is $2.9 billion.

We have not yet arranged employment for our three seventh generation hulls under construction, which are scheduled to be delivered in July 2013, October 2013 and November 2013, respectively.

In addition, on February 9, 2012, Petrobras Brazil announced it awarded 15-year term charters to a consortium in which we are a participant, for five ultra-deepwater drilling units at an average day rate of $548,000. We are considering our options with respect to our possible participation in this project, as no agreement has been signed to date.

Management of Our Fleet

Our wholly-owned subsidiary, Ocean Rig AS, provides supervisory management services including onshore management, to our operating drilling rigs and drillships pursuant to separate management agreements entered into with each of the drilling unit-owning subsidiaries. Ocean Rig AS also provides supervisory management services for our seventh generation hulls under construction.

Under the terms of these management agreements, Ocean Rig AS, through its offices in Stavanger, Norway, Aberdeen, United Kingdom and Houston, Texas, is responsible for, among other things, (i) assisting in construction contract technical negotiations, (ii) securing contracts for the future employment of the drillships, and (iii) providing commercial, technical and operational management for the drillships.

Pursuant to the Global Services Agreement between DryShips and Cardiff, a company controlled by our Chairman, President and Chief Executive Officer, Mr. George Economou, effective December 21, 2010, DryShips has engaged Cardiff to act as consultant on commercial, financial and corporate matters for the offshore drilling units operated by us. Under the Global Services Agreement, Cardiff, or its subcontractor, will (i) provide consulting services related to identifying, sourcing, negotiating and arranging new employment for offshore assets of DryShips and its subsidiaries, including our drilling units and (ii) identify, source, negotiate and arrange the sale or purchase of the offshore assets of DryShips and its subsidiaries, including our drilling units. In consideration of the services provided by Cardiff under the Global Services Agreement, DryShips will pay Cardiff a fee of 1.0% in connection with employment arrangements and 0.75% in connection with sale and purchase activities. We do not pay for services provided in accordance with this agreement. The costs of services we receive under the Global Services Agreement are expensed in our consolidated statement of operations or capitalized as directly attributable to construction costs under “Advances for rigs and drillships under construction”. The payment by DryShips for services provided to us under the agreement is deemed an equity contribution to us and is recorded as shareholders’ contribution to shareholders’ equity. See “Item 7. Major Shareholders and Related Party Transactions—B. Related party transactions—Global Services Agreement” in our Annual Report on Form 20-F for the year ended December 31, 2011 incorporated by reference herein.

Our Competitive Strengths

We believe that our growth prospects are enhanced by the following aspects of our business:

Proven track record in ultra-deepwater drilling operations.    We have a well-established record of operating drilling equipment with a primary focus on ultra-deepwater offshore locations and harsh environments. Established in 1996, we employed 1,305 people as of December 31, 2011, and have gained significant experience operating in challenging environments with a proven operating track record through our completion of 122 wells. We capitalize on our high-specification drilling units to the maximum extent of their technical capability, and we believe that we have earned a reputation for operating performance excellence. We have operated the Leiv Eiriksson since 2001 and the Eirik Raude since 2002.

Technologically advanced deepwater drilling units.    According to Fearnley Offshore AS, the Leiv Eiriksson and the Eirik Raude are two of only 15 drilling units worldwide as of March 2012 that are technologically equipped to operate in both ultra-deepwater and harsh environments. Additionally, each of our drillships will be either a sixth or seventh generation, advanced capability, ultra-deepwater drillship built based on a proven design that features full dual derrick enhancements. Our operating drillships have the capacity to drill 40,000 feet at water depths of 10,000 feet and our seventh generation hulls will have the capacity to drill 40,000 feet at water depths of 12,000 feet. One of the key benefits of each of our drillships is its dual activity drilling capabilities, which involves two drilling systems that use a single derrick and which permits two drilling-related operations to take place simultaneously. We estimate that this capability reduces typical drilling time by approximately 15% to 40%, depending on the well parameters, resulting in greater utilization and cost savings to our customers. According to Fearnley Offshore AS, of the 29 ultra-deepwater drilling units delivered worldwide in 2011, only 13 have dual activity drilling capabilities, including our four operating drillships. As a result of the Deepwater Horizon offshore drilling accident in the Gulf of Mexico in April 2010, in which we were not involved, we believe that independently and nationally owned oil companies and international governments

will increase their focus on safety and the prevention of environmental disasters and, as a result, we expect that high quality and technologically advanced drillships such as ours will be in high demand and at the forefront of ultra-deepwater drilling activity.

Long-term blue-chip customer relationships.    Since the commencement of our operations in 1996, we have developed relationships with large independent oil and gas companies such as Chevron Corporation, or Chevron, Exxon Mobil Corporation, or ExxonMobil, Petrobras Oil & Gas, Royal Dutch Shell plc, or Shell, BP plc, or BP, Total S.A., or Total, Statoil ASA, or Statoil, and Tullow Oil. Together with our predecessor, Ocean Rig ASA, we have drilled 122 wells in 13 countries for 23 clients, including those listed above. Currently, we have employment contracts with Petrobras Oil & Gas, Petrobras Tanzania, Tullow Oil, Borders & Southern, Anadarko and Vanco. We believe these strong customer relationships stem from our proven track record of dependability and of delivering high-quality drilling services in the most extreme operating environments. Although our former clients are not obligated to use our services, we expect to use our relationships with our current and former customers to secure attractive employment contracts for our drilling units.

High barriers to entry.    There are significant barriers to entry in the ultra-deepwater offshore drilling industry. Given the technical expertise needed to operate ultra-deepwater drilling rigs and drillships, operational know-how and a track record of safety play an important part in contract awards. The offshore drilling industry in some jurisdictions is highly regulated, and compliance with regulations requires significant operational expertise combined with financial and management resources. With the recent negative press around the Deepwater Horizon drilling rig accident, we expect regulators worldwide to implement more stringent regulations and oil companies to place a premium on drilling firms with a proven track record for safety and more modern units. There are also significant capital requirements for building ultra-deepwater drillships. Further, there is limited shipyard availability for new drillships and required lead times are typically in excess of two years. Additionally, due to the ongoing global financial stresses, access to bank lending, the traditional source for ship and offshore financing, has become constrained. According to Fearnley Offshore AS, as of March 2012, there were 112 ultra-deepwater drilling units in operation with another 78 under construction, including our three newbuilding drillships.

Anticipated strong free cash flow generation.    Based on current and expected supply and demand dynamics in ultra-deepwater drilling, we expect dayrates to be above our estimated daily cash breakeven rate, based on estimated daily operating costs, general and administrative costs and scheduled debt service requirements, thereby generating substantial free cash flow going forward. According to Fearnley Offshore AS, the most recent charterhire in the industry for a modern ultra-deepwater drillship or rig (February 2012) was at a gross dayrate of $585,000 for an 18-month contract commencing in the first quarter of 2013. The maximum dayrates for the contracts on which our drilling units were employed during February 2012 ranged between $498,000 to $586,287 per day.

Leading shipbuilder constructing our newbuildings.    Only a limited number of shipbuilders possess the necessary construction and underwater drilling technologies and experience to construct and operate drillships. Our operating drillships were and our three newbuilding drillships are being built by Samsung, which is one of the world’s largest shipbuilders in the high-tech and high-value shipbuilding sectors. This includes drillships, ultra-large container ships, liquefied natural gas carriers and floating production storage and offshore units, or FPSOs. According to Fearnley Offshore AS, of the 109 drillships ordered on a global basis from

2005, Samsung has delivered or will deliver 44, representing a 40% market share. To date, construction of our newbuilding drillships has progressed on time and on budget.

Experienced management and operations team.    We have an experienced management and operations team with a proven track record and an average of 21 years of experience in the offshore drilling industry. Many of the core members of our management team have worked together since 2006, and certain members of our management team have worked at leading oil-related and shipping companies such as ExxonMobil, Transocean Ltd., Petrofac, Aker Kvaerner and Sevan Drilling. In addition to the members of the management team, we had at December 31, 2011, 30 people overseeing construction of our newbuilding drillships and highly trained personnel operating the drillships. We also had at December 31, 2011 an onshore team of 124 people in management functions as well as administrative and technical staff and support functions, ranging from marketing, human resources, accounting, finance, technical support and health, environment, safety and quality, or HES&Q. We believe the focus and dedication of our personnel in each step of the process, from design to construction to operation, has contributed to our track record of safety and consistently strong operational performance.

Business Strategy

Our business strategy is predicated on becoming a leading company in the offshore ultra-deepwater drilling industry and providing customers with safe, high quality service and state-of-the-art drilling equipment. The following outlines the primary elements of our strategy:

Participate in favorable drilling industry and ultra-deepwater sector dynamics.    We believe the demand for offshore deepwater drilling units will be positively affected by increasing global demand for oil and gas and increased exploration and development activity in deepwater markets. The International Energy Agency, or the IEA, projected that oil demand for 2010 increased by 3.2% compared to 2009 levels, and as of October 2011 oil demand had increased by 1.1% compared to 2010 levels. As the Organization for Economic Co-operation and Development, or OECD, countries resume their growth and the major non-OECD countries continue to develop, led by China and India, oil demand is expected to grow. We believe it will become increasingly difficult to find the incremental barrels of oil needed, due to depleting existing oil reserves. This is expected to force oil companies to continue to explore for oil farther offshore to grow their proven reserves. According to Fearnley Offshore AS, from 2005 to 2010, the actual spending directly related to ultra-deepwater drilling units increased from $4.7 billion to $19.0 billion, a compound average growth rate, or CAGR, of 32.2%.

Maintain a “pure play” focus in the ultra-deepwater and harsh environment markets.    Our mission is to become the preferred offshore drilling contractor in the ultra-deepwater and harsh environment regions of the world and to deliver excellent performance to our clients by exceeding their expectations for operational efficiency and safety standards. We believe our operating drillships are, and our three newbuilding drillships will be, among the most technologically advanced in the world. We currently have an option to purchase up to three additional newbuilding drillships and we intend to grow our fleet over time in order to continue to meet our customers’ demands while optimizing our fleet size from an operational and logistical perspective.

Capitalize on the operating capabilities of our drilling units.    We plan to capitalize on the operating capabilities of our drilling units by entering into attractive employment contracts. The

focus of our marketing effort is to maximize the benefits of the drilling units’ ability to operate in ultra-deepwater drilling locations. As described above, the Leiv Eiriksson and Eirik Raude are two of only 15 drilling units worldwide as of March 2012 that are technologically equipped to operate in both ultra-deepwater and harsh environments, and our drillships will have the capacity to drill 40,000 feet at water depths of 10,000 feet or, in the case of our seventh generation hulls, 12,000 feet with dual activity drilling capabilities. We aim to secure firm long-term employment contracts for the drilling units at or near the highest dayrates available in the industry at that time. As we work towards our goal of securing firm contracts for our drilling units at attractive dayrates, we believe we will be able to differentiate ourselves based on our prior experience operating drilling rigs and our safety record.

Maintain high drilling unit utilization and profitability.    We have a proven track record of optimizing equipment utilization. We aim to maximize the revenue generation of our drilling units by maintaining our track record of high drilling unit utilization as a result of the design capabilities of our drilling units that can operate in harsh environmental conditions. Despite operating under severely harsh weather conditions, we have a proven track record of high efficiency deepwater and ultra-deepwater drilling operations. We employed 1,305 people as of December 31, 2011 and have been operating ultra-deepwater drilling rigs since 2001. We have extensive experience working in varying environments and regulatory regimes across the globe, including Eastern Canada, Angola, Congo, Ireland, the Gulf of Mexico, the U.K., West of Shetlands, Norway, including the Barents Sea, Ghana, Turkey, Tanzania and the Ivory Coast.

Implement and sustain a competitive cost structure.    We believe that we have a competitive cost structure due to our operating experience and successful employee retention of highly-skilled personnel which leads to significant transferable experience and knowledge of drilling rig operation through deployment of seasoned crews across our fleet. By focusing on the ultra-deepwater segment, we believe that we are able to design and implement best-in-class processes to streamline our operations and improve efficiency. As we grow, we intend to benefit from significant economies of scale due to an increased fleet size and a fleet of “sister-ships” to our drillships, where we expect to benefit from the standardization of these drilling units, resulting in lower training and operating costs. In addition, our drillships have high-end specifications, including advanced technology and safety features, and, therefore, we expect a limited need for upgrades in the near term. We expect the increase from six to nine drilling units to enable us to bring more than one unit into a drilling region in which we operate, which would provide benefits from economies of scale and improved logistic coordination managing more units from the same onshore bases.

Continue to prioritize safety as a key focus of our operations.    We believe safety is of paramount importance to our customers and a key differentiator for us when securing drilling contracts from our customers. We have a zero incident philosophy embedded in our corporate culture, which is reflected in our policies and procedures. Despite operating under severely harsh weather conditions, we have a proven track record of high efficiency deepwater and ultra-deepwater drilling operations. We employed 1,305 people as of December 31, 2011 and have been operating ultra-deepwater drilling rigs since 2001. We have extensive experience working in varying environments and regulatory regimes across the globe, including Eastern Canada, Angola, Congo, Ireland, the Gulf of Mexico, the U.K., West of Shetlands, Norway, including the Barents Sea, Ghana, Turkey, Tanzania and the Ivory Coast.

One of our drilling rigs and one of our drillships, the Ocean Rig Corcovado, have a valid and updated safety case under U.K. Health and Safety Executive, or HSE, regulations, and both of

our drilling rigs hold a Norwegian sector certificate of compliance (called an Acknowledgement of Compliance), which evidences that the rigs and our management system meet the requirements set by the U.K. and Norwegian authorities. We believe that this good safety record has enabled us to hire and retain highly-skilled employees, thereby improving our overall operating and financial performance. We expect to continue our strong commitment to safety across all of our operations by investing in the latest technologies, performing regular planned maintenance on our drilling units and investing in the training and development of new safety programs for our employees.

Industry Overview

In recent years, the international drilling market has seen an increasing trend towards deep and ultra-deepwater oil and gas exploration. As shallow water resources mature, deep and ultra-deepwater regions are expected to play an increasing role in offshore oil and gas exploration and production. According to Fearnley Offshore AS, the industry-wide global ultra-deepwater market has seen rapid development over the last six years, with dayrates increasing from approximately $180,000 in 2004 to above $600,000 in 2008. More recently, the market declined to an average level of approximately $535,000 in March 2012. The ultra-deepwater market rig utilization rate has been stable above 80% since 2000 and above 97% since 2006. The operating units capable of drilling in ultra-deepwater depths of greater than 7,500 feet consist mainly of fifth- and sixth-generation units, but also include certain older upgraded units. The in-service fleet as of March 2012 totaled 112 units, and is expected to grow to 190 units upon the scheduled delivery of the current newbuild orderbook by the end of 2020. Historically, an increase in supply has caused a decline in utilization and dayrates until drilling units are absorbed into the market. Accordingly, dayrates have been very cyclical. We believe that the largest undiscovered offshore reserves are mostly located in ultra-deepwater fields and primarily located in the “golden triangle” between West Africa, Brazil and the Gulf of Mexico, as well as in East Africa, Australia and Southeast Asia. The location of these large offshore reserves has resulted in more than 90% of the floater orderbook being represented by ultra-deepwater units. Furthermore, due to increased focus on technically challenging operations and the inherent risk of developing offshore fields in ultra-deepwater, particularly in light of the Deepwater Horizon accident in the Gulf of Mexico, oil companies have already begun to show a preference for modern units more capable of drilling in these challenging environments. See “The Offshore Drilling Industry.”

Dividend Policy

Our long-term objective is to pay a regular dividend in support of our main objective to maximize shareholder returns. However, we have not paid any dividends in the past and we are currently focused on the development of capital intensive projects in line with our growth strategy and this focus will limit any dividend payment in the medium term. Because we are a holding company with no material assets other than the shares of our subsidiaries through which we conduct our operations, our ability to pay dividends will depend on our subsidiaries distributing their earnings and cash flow to us. In addition, under our $800.0 million senior secured term loan agreement, which matures in 2016, we are prohibited from paying dividends without the consent of our lenders. Furthermore, our $800.0 million senior secured term loan agreement and our two Deutsche Bank credit facilities limit or prohibit our subsidiaries’ ability to make distributions without the consent of our lenders. We have, however, received the required consent from our lenders, subject to definitive documentation, to amend our credit facilities, under which we would be permitted to pay dividends of up to 50% of our net income of each previous year provided that following each payment, we maintain certain levels of minimum cash and cash equivalents.

Any future dividends declared will be at the discretion of our board of directors and will depend upon our financial condition, earnings and other factors, including the financial covenants contained in our loan agreements and our 9.5% senior unsecured notes due 2016. Our ability to pay dividends is also subject to Marshall Islands law, which generally prohibits the payment of dividends other than from operating surplus or while a company is insolvent or would be rendered insolvent upon the payment of such dividend.

Corporate Structure

Ocean Rig UDW Inc. is a corporation incorporated under the laws of the Republic of the Marshall Islands on December 10, 2007 under the name Primelead Shareholders Inc. Primelead Shareholders Inc. was formed in December 2007 for the purpose of acquiring the shares of our predecessor, Ocean Rig ASA, which was incorporated in September 1996 under the laws of Norway. Ocean Rig UDW acquired control of Ocean Rig ASA on May 14, 2008. Prior to the private placement of our common shares in December 2010, we were a wholly-owned subsidiary of DryShips. As of the date of this prospectus, DryShips owns approximately 73.9% of our outstanding common shares. Following this offering, DryShips will own approximately 67.0% of our common shares (or 66.0% if the underwriters exercise their over-allotment option in full). Each of our drilling units is owned by a separate wholly-owned subsidiary.

We maintain our principal executive offices at 10 Skopa Street, Tribune House, 2^nd Floor, Office 202, CY 1075, Nicosia, Cyprus and our telephone number at that address is 011 357 22767517. Our website is located at www.ocean-rig.com. The information on our website is not a part of this prospectus.

Recent Developments

Future Drilling Contracts; Newbuilding Option Extension; Interest Rate Swaps

In January 2012, we entered into a new drilling contract for the Eirik Raude with Ophir Services at a maximum dayrate of $647,651, which may be increased to $668,984 in the event we are required to recruit certain additional crew, for the drilling of three wells offshore Equatorial Guinea. In addition, we are entitled to a mobilization fee of $568,400 per day, plus the cost of fuel, and a demobilization fee of $4.75 million; however, we are not entitled to the demobilization fee in the event the Eirik Raude engages in drilling operations with other companies in direct continuation of the contract with Ophir Services. The contract has an estimated duration of 60 days into the third quarter of 2012 and Ophir Services has the option to extend the contract term by one additional well, which is estimated to extend the duration of the contract by an additional 20 days.

On February 2 and February 9, 2012, we entered into two interest rate swap agreements for a total notional amount of $700.0 million maturing in April 2016.

On February 9, 2012, Petrobras Brazil announced it awarded 15-year term charters to a consortium in which we are a participant, for five ultra-deepwater drilling units at an average day rate of $548,000. We are considering our options with respect to our possible participation in this project, as no agreement has been signed to date.

In February 2012, we entered into a new drilling contract for the Leiv Eiriksson with a consortium coordinated by Rig Management Norway for the drilling of 15 wells on the Norwegian Continental Shelf at a maximum dayrate of $545,000, plus a mobilization fee of $26.0 million. The contract has a minimum duration of 1,070 days and includes three options of up to 6 wells each that must be exercised prior to the expiry of the firm contract period.

In February 2012, we agreed with Tullow Ghana to extend the number of operational days under the contracts for the Ocean Rig Olympia from 108 to 120 and increase the dayrate to $600,000 per day in the event drilling operations occur after April 7, 2012. The contracts are scheduled to expire at the end of May 2012.

In March 2012, we entered into a new drilling contract for the Eirik Raude with European Hydrocarbons for the drilling of two wells offshore West Africa at a maximum dayrate of $718,750, which includes the operating dayrate of $625,000 plus the maximum performance bonus available under the contract. The operating rate may be increased to $671,875 in the event we are unable to use certain crew that were on the rig under the contract immediately prior to our contract with European Hydrocarbons or we are required to recruit certain additional crew, in which case the maximum dayrate available under the contract, including the maximum performance bonus, would be $772,656. We are also entitled to a mobilization fee under the contract of $15.0 million plus the cost of fuel consumed during the mobilization period. The contract has an estimated duration of 84 days and the customer has the option to extend the contract for (i) such number of days for the testing of one additional well at the applicable operating rate or (ii) the drilling of one additional well, which is estimated to extend the duration of the contract by 40 days, for a fee of approximately $24.9 million. The contract is scheduled to commence following the completion of the contract with Ophir Services in the fourth quarter of 2013.

On March 22, 2012, the Ocean Rig Mykonos completed the general testing of equipment required under the contract with Petrobras Brazil and subsequently commenced drilling operations under the contract.

In April 2012, we announced the receipt of a letter of award from Total for a three-year contract for drilling operations offshore West Africa for the Ocean Rig Olympia, to commence upon the completion of the contract with Tullow Ghana, at a maximum dayrate of $584,450, plus mobilization and demobilization fees of $9.0 million and $3.5 million, respectively, plus the cost of fuel. Under the contract, Total will have the option to extend the term of the contract for two additional one-year periods at the dayrate specified above, adjusted annually for inflation, with the first option exercisable within one year from the commencement date under the drilling contract, and the second option exercisable within one year after the date of exercise of the first option. The contract is subject to final documentation and the satisfaction of customary conditions precedent, which we expect to complete during April 2012. The letter of award expires on April 30, 2012.

On April 2, 2012, we entered into an addendum to our option contract with Samsung to extend the deadline for exercising the remaining three options for the construction of up to three additional seventh generation ultra-deepwater drillships from April 2, 2012 to October 4, 2012, with the first vessel being delivered in the fourth quarter of 2014 and the second and third vessels being delivered on the earliest available delivery dates based on the production schedule, as determined by Samsung in its reasonable discretion.

Loan Facility Amendments

On March 27, 2012, we received the required consent of our lenders under our $1.04 billion credit facility to amend the facility to remove the cross-acceleration clause relating to DryShips’s indebtedness. As a result of the amendment, a default by DryShips under one of its loan agreements and the acceleration of the related debt will no longer result in a cross-default under our $1.04 billion credit facility that would provide our lenders with the right to accelerate the outstanding debt under the facility. Under the terms of the facility, as amended, the cross-acceleration clause relating to DryShip’s outstanding indebtedness will be replaced with a cross-acceleration relating to the outstanding indebtedness of Ocean Rig UDW Inc., which will provide a guarantee under the facility. The amendment is subject to definitive documentation and the satisfaction of certain conditions precedent.

On April 3, 2012, we received the required consent of our lenders under our $800.0 million secured term loan agreement to amend the loan agreement to, among other things, terminate the guarantee by DryShips, remove the related covenants and the cross-acceleration provision relating to DryShips’s indebtedness. As a result of the amendment, a default by DryShips under one of its loan agreements and the acceleration of the related debt will no longer result in a cross-default under our $800.0 million secured term loan agreement that would provide our lenders with the right to accelerate the outstanding debt under the loan agreement. In addition, under the terms of the loan agreement, as amended, (i) the Company will be permitted to buy-back its own common shares; (ii) Drillships Holdings Inc. will be able to pay dividends to us as its shareholder; and (iii) we will be permitted to pay dividends to our shareholders of up to 50% of our net income of each previous financial year, provided in each case that we maintain minimum free cash of $200.0 million and maintain such level for the next 12 months. The amendments will also provide for a reduction in the amount of minimum free cash required to be maintained by Drillships Holdings Inc. from $75.0 million to $50.0 million. Under the agreement, we are required to maintain minimum free cash of $100 million. The amendments are subject to definitive documentation and the satisfaction of certain conditions precedent, including the entry into corresponding amendments to our Deutsche Bank credit facilities.

On April 4, 2012, we received the required consent of our lenders under our Deutsche Bank credit facilities to amend the facilities to, among other things, terminate the DryShips guarantee and remove the related covenants and the cross-default provision relating to DryShips’s outstanding indebtedness. As a result of the amendment, a default by DryShips under one of its loan agreements will no longer result in a cross-default under our Deutsche Bank credit facilities that would provide our lenders with the right to accelerate the outstanding debt under these facilities. In addition, the amendments will also remove the cash sweep repayment mechanism under the facilities. We will also be required to build up a reserve account of $57 million by September 2014. Furthermore, under the facilities, as amended, we will be permitted to pay dividends, make distributions and effect redemptions or returns of share capital provided we maintain minimum liquidity in an aggregate amount of not less than $200.0 million in free cash and maintain such level for the next 12 months. Under the terms of the facilities, as amended, in the event of a breach by us of the financial covenants contained in our guarantee, DryShips will be required to issue a new unconditional and irrevocable payment guarantee in favor of the security trustee, substantially in the same form as the guarantee currently in place under the facilities; however, such guarantee will not include any financial covenants applicable to DryShips or cross-default provisions in relation to DryShips’s indebtedness. The amendments are subject to definitive documentation and the satisfaction of certain conditions precedent.

Risk Factors

We face a number of risks associated with our business and industry and must overcome a variety of challenges to utilize our strengths and implement our business strategy. These risks include, among others, changes in the offshore drilling market, including supply and demand, utilization rates, dayrates, customer drilling programs, and commodity prices; increased costs of compliance with regulations affecting the offshore drilling industry; a downturn in the global economy; hazards inherent in the drilling industry and marine operations resulting in liability for personal injury or loss of life, damage to or destruction of property and equipment, pollution or environmental damage; inability to comply with loan covenants; inability to finance shipyard and other capital projects; and inability to successfully employ our drilling units.

This is not a comprehensive list of risks to which we are subject, and you should carefully consider all the information in this prospectus in connection with your ownership of our common shares. In particular, we urge you to carefully consider the risk factors set forth in the section of this prospectus entitled “Risk Factors” beginning on page 22 and under the heading “Risk Factors” in our Annual Report on Form 20-F for the year ended December 31, 2011, filed on March 14, 2012.

The Offering

Summary Financial Information

The following table sets forth our summary historical consolidated financial and other data, at the dates and for the periods indicated. We were incorporated on December 10, 2007 under the name Primelead Shareholders Inc. Primelead Shareholders Inc. was formed for the purposes of acquiring the shares of our predecessor, Ocean Rig ASA, which was incorporated in September 1996 under the laws of Norway. We acquired control of Ocean Rig ASA on May 14, 2008. The summary historical consolidated financial data as of December 31, 2011 and 2010 and for the years ended December 31, 2011, 2010 and 2009 is derived from the audited financial statements and related notes of Ocean Rig UDW Inc. and its subsidiaries (“successor”) included in our Annual Report on Form 20-F for the year ended December 31, 2011 incorporated by reference into this prospectus.

The summary historical consolidated financial data as of December 31, 2009 and 2008 and for the year ended December 31, 2008 is derived from the audited financial statements and related notes of Ocean Rig UDW Inc. and its subsidiaries (“successor”) not included in our Annual Report on Form 20-F for the year ended December 31, 2011. The summary historical consolidated financial and other data of Ocean Rig ASA and its subsidiaries (“predecessor”) as of and for the period from January 1 to May 14, 2008 and for the year ended December 31, 2007 is derived from the audited financial statements of Ocean Rig ASA not included in our Annual Report on From 20-F for the year ended December 31, 2011.

We refer you to the notes to the consolidated financial statements for a discussion of the basis on which the consolidated financial statements are presented. The summary historical consolidated financial and other data should be read in conjunction with “Item 5. Operating and Financial Review and Prospects” and the audited consolidated financial statements, the related notes thereto and other financial information included in our Annual Report on Form 20-F for the year ended December 31, 2011 incorporated by reference into this prospectus.