UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D. C. 20549

FORM 10-K/A

(Amendment No.1)

For the fiscal year ended December 31, 2011

or

Commission file number: 001-32647

ATP Oil & Gas Corporation

(Exact name of registrant as specified in its charter)

4600 Post Oak Place, Suite 100

Houston, Texas 77027

(Address of principal executive offices) (Zip Code)

Registrant’s telephone number, including area code: (713) 622-3311

Securities Registered Pursuant to Section 12 (b) of the Act:

Securities Registered Pursuant to Section 12 (g) of the Act: None

Indicate by check mark if the Registrant is a well-known seasoned issuer as defined in Rule 405 of the Securities Act.     Yes  ¨    No  x

Indicate by check mark if the Registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.     Yes  ¨    No  x

Indicate by check mark whether the Registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the Registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the Registrant has submitted electronically and posted on its corporate website, if any, every interactive Data File required to be submitted electronically and posted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or such shorter period that Registrant was required to submit and post such files).    Yes  x    No  ¨

        Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of Registrant’s knowledge, in definitive proxy or information statements incorporated by Reference in Part III of this Form 10-K or any amendment to this Form 10-K.  ¨

Indicate by check mark whether the Registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the Registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

The aggregate market value of the voting and non-voting common stock held by non-affiliates of the Registrant as of June 30, 2011 (the last business day of the Registrant’s most recently completed second fiscal quarter) was approximately $682.1 million. The number of shares of the Registrant’s common stock outstanding as of March 2, 2012 was 52,051,422.

DOCUMENTS INCORPORATED BY REFERENCE

Selected portions of ATP Oil & Gas Corporation’s definitive Proxy Statement, which will be filed with the Securities and Exchange Commission within 120 days after December 31, 2011, are incorporated by reference in Part III of this Form 10-K.

EXPLANATORY NOTE

ATP Oil & Gas Corporation (the “Company”) is filing this Amendment No. 1 on Form 10-K/A (“Amendment No. 1”) solely to include XBRL (eXtensible Business Reporting Language) information in Exhibit 101 that was excluded from our filed Annual Report on Form 10-K for the fiscal year ended December 31, 2011. This Amendment No. 1 makes no other changes to the Annual Report on Form 10-K as filed with the Securities and Exchange Commission on March 15, 2011.

Exhibit 101 provides the following items formatted in XBRL: (i) Balance Sheets at December 31, 2011 and December 31, 2010; (ii) Statements of Operations for the fiscal years ended December 31, 2011, December 31, 2010 and December 31, 2009; (iii) Statements of Comprehensive Income (Loss) for the fiscal years ended December 31, 2011, December 31, 2010 and December 31, 2009; (iv) Statements of Cash Flows for the fiscal years ended December 31, 2011, December 31, 2010 and December 31, 2009; (v) Statements of Temporary Equity for the fiscal years ended December 31, 2011, December 31, 2010 and December 31, 2009; (vi) Statements of Shareholders’ Equity for the fiscal years ended December 31, 2011, December 31, 2010 and December 31, 2009; (vii) Notes to Consolidated Financial Statements; (viii) Schedule I – Parent Company Financial Statements; and (ix) Schedule II – Valuation and Qualifying Accounts.

EXHIBITS

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this amendment to be signed on its behalf by the undersigned, thereunto duly authorized, on March 23, 2012.