Exhibit 10.15

WAIVER AGREEMENT

This WAIVER AGREEMENT is dated as of August 16, 2012 (this “Agreement”) by and among Plains Offshore Operations Inc., a Delaware corporation (“Company”), PXP Resources LLC, a Delaware limited liability company (“PXP LLC”), Plains Exploration & Production Company, a Delaware corporation (together with any Affiliate of Plains Exploration & Production Company, “PXP”), and each Investor named on the signature pages hereto. The parties to this Agreement are collectively referred to herein as the “Parties” Capitalized terms not defined herein have the meaning ascribed to them in the Stockholders Agreement (as defined below).

WHEREAS, the Parties are party to that certain Stockholders Agreement dated November 17, 2011, as amended by Amendment No. 1 thereto dated as of December 22, 2011 and Amendment No. 2 thereto dated as of January 31, 2012 (the “Stockholders Agreement”);

WHEREAS, PXP proposes to purchase certain Deepwater GOM Assets, including all or certain interests of BP Exploration & Production Inc. and BP America Production Company in and near the Holstein, Diana, Hoover, Horn Mountain, Ram Powell, and Marlin Hub Fields, and certain interests of Royal Dutch Shell plc in and near the Holstein Field, including but not limited to the following offshore blocks: Alaminos Canyon 25 and 26, East Breaks 945, 946 and 989, Green Canyon 644, 645 and 688, Mississippi Canyon 82, 84, 85, and 126 - 129, and Viosca Knoll 871, 911 - 913, 915, and 955 - 957 (collectively, the “GOM Assets”);

WHEREAS, pursuant to Section 2.02(c) of the Stockholders Agreement, in the event that PXP acquires the GOM Assets, PXP is required to offer to sell the GOM Assets to the Company within thirty (30) days after such acquisition (the closing date of PXP’s acquisition of the GOM Assets is referred to herein as the “GOM Asset Closing Date”); and

WHEREAS, the Parties desire to set forth the terms under which the Company will waive its right to purchase the GOM Assets under the Stockholders Agreement.

NOW THEREFORE, the Parties hereby agree as follows:

1. Waiver of Right to Purchase. Notwithstanding the provisions of Section 2.02(c) of the Stockholders Agreement, the Parties hereby agree that PXP may bid for and acquire the GOM Assets; provided that PXP shall provide to the Company notice of any such proposed acquisition not less than seventy-five (75) days prior to the GOM Asset Closing Date, which notice shall contain a summary of the terms on which PXP proposes to acquire the GOM Assets (the “PXP Notice”). The Company shall have the right to elect to purchase the GOM Assets in full, but not in part, on the terms set forth in the PXP Notice if, and only if, the Investors (by Majority Preferred Approval) approve such acquisition from PXP and the Company can provide a financing plan for its acquisition that is agreed upon by PXP and the Investors (by Majority Preferred Approval) within sixty (60) days after receipt of the PXP Notice. In the event the Company acquires the GOM Assets, any equity component of the approved financing plan shall be funded by PXP and the Investors through purchases of Class A Common Stock, Class B Common Stock or Common Stock, as applicable, in accordance with the terms of Section 2.02(c) of the Stockholders Agreement as though such acquisition were an acquisition of Deepwater GOM Assets by Company elected in accordance with Section 2.02(c) of the Stockholders Agreement unless the

Parties agree otherwise. The Company shall use commercially reasonable efforts to arrange such financing plan on the best terms available in the market. Such financing plan may require, among other things, amendments to the Credit Agreement and the Company’s dividend policy set forth in the Certificate of Designation if agreed to by PXP and Majority Preferred Approval. Without limiting the ability of the parties to agree on a financing plan on other terms, if senior debt financing for the acquisition can be obtained by the Company on terms consistent with customary industry borrowing based standards, and PXP approval and Majority Preferred Approval is obtained for such financing plan (including the equity component thereof) within said sixty (60) day period the Company shall use commercially reasonable efforts to obtain all other approvals for such financing plan and to exercise the right to purchase the GOM Assets. If a financing plan cannot be agreed upon by PXP and the Investors within said sixty (60) day period and any GOM Assets are acquired by PXP rather than the Company within 180 days of the date hereof, the Company shall be deemed to have waived any interest in, or right to purchase from PXP, any of the GOM Assets and the provisions of Section 2 hereof shall automatically operate. The Company shall provide the Investors such information as they may reasonably request from time to time regarding the terms of the acquisition or the financing plan, including periodic updates on status of the Company’s efforts to arrange such financing. PXP shall cooperate in efforts to arrange the financing plan and any acquisition of the GOM Assets by the Company in accordance with the terms of this Section 1. The provisions of this Section 1 shall be deemed limited to an acquisition of GOM Assets that occurs within 180 days of the date hereof.

2. In accordance with the provisions of Section 1 above, if a financing plan cannot be agreed upon by PXP and the Investors and any GOM Assets are acquired by PXP rather than the Company within 180 days of the date hereof, the Parties hereby amend the Stockholders Agreement as follows:

(kkk) “AMI Assets” means any right, title or interest, of any kind or character, in those certain deepwater Gulf of Mexico blocks currently owned by the Company including those set forth on Schedule B at all depths (the “Subject Blocks”) and those deepwater Gulf of Mexico blocks within a one-block contiguous halo of each Subject Block to the extent that said contiguous halo blocks are identified with potential Pliocene-aged reservoirs (including in each case the equity interests of a company that owns any such assets).

(c) Exclusivity. PXP may acquire any AMI Assets, provided that, within thirty (30) days after the closing of such acquisition, PXP offers, by written notice, to sell such assets to the Company for an amount equal to the purchase price of such assets plus PXP’s reasonable transaction costs. The Company shall notify

2

PXP as to whether it will acquire such AMI Assets within thirty (30) days after receipt of a written offer from PXP. The Company shall take all action necessary or appropriate to seek the vote or consent to approve such offer by the holders of Preferred Stock and, prior to an Initial Public Offering, the holders of Common Shares, each voting separately as a class (except that the holders of Common Shares shall vote together as a single class), and in connection therewith shall provide all such holders with all material information relating to such offer and such assets. PXP agrees, and agrees to cause its Affiliates, to vote or consent all of their respective Common Shares (if such vote or consent is sought prior to an Initial Public Offering and determined by the Company to be necessary) and Preferred Stock entitled to vote or consent to approve such offer in the same proportion as the holders of the Preferred Stock other than PXP or its Affiliates, voting separately as a class, not later than three (3) days after the vote or consent thereon by the holders of the Preferred Stock other than PXP or its Affiliates. The Company shall accept such offer if (and only if) it has obtained: (i) prior to an Initial Public Offering, the Majority Preferred Approval (without any further requirement of approval by the Board of Directors); and (ii) after an Initial Public Offering, the approval of the Board of Directors. If the Company agrees to acquire any AMI Assets prior to an Initial Public Offering under this Section 2.02(c), then each of the holders of Common Stock, Class A Common Stock and Class B Common Stock (the “Common Stockholders”) and Preferred Stockholders shall contribute its proportionate share (determined on an as-converted basis) of any capital necessary to fund such acquisition through the purchase from the Company of additional shares of Class A Common Stock at a price of $20.00 per share; provided, however, that notwithstanding anything to the contrary in Section 3.08, any or all of MTP Energy Master Fund Ltd., MTP Energy Opportunities Fund I LLC, MTP Opportunity Fund, LLC, Triangle Peak Partners Private Equity, LP and Hipparchus Fund LP (each, together with its Affiliates, a “Magnetar Fund”) or FSEP Term Funding, LLC, FS Investment Corporation, and Locust Street Funding LLC (each, together with its Affiliates, a “GSO Fund”), may assign, in whole or in part (including without limitation an assignment limited to a particular offer under this Section 2.02(c)), its rights and (to the extent related to such assigned rights) obligations under this Section 2.02(c), in their capacity as holders of Preferred Stock, to acquire additional shares of Class A Common Stock to one or more of Energy Fund XV, L.P., Energy Fund XV-A, L.P., Energy Fund XV-B, L.P. and Energy XV Blocker (Plains), LLC (each, together with its Affiliates, an “EIG Fund” and collectively “EIG”), subject to the assignee EIG Fund assuming such assigned obligations under this Section 2.02(c) and notifying the Company of such assignment and assumption (and specifying the rights and obligations assigned and assumed) and (1) to the extent an EIG Fund assumes such obligations under this Section 2.02(c), the Magnetar Fund or GSO Fund assigning such obligations under this Section 2.02(c) shall be relieved of such rights and obligations under this Section 2.02(c) and (2) the rights and obligations so assigned and assumed shall be deemed to be the rights and obligations of the EIG Fund that has assumed such obligations; provided further, however, that if any holder of Common Stock,

3

Class A Common Stock or Class B Common Stock fails to purchase such additional shares of Class A Common Stock, then PXP shall acquire such additional shares of Class A Common Stock. If the foregoing applicable approval is obtained, then the Company shall (and PXP shall cause the Company to) take all action necessary or appropriate to accept such offer within thirty (30) days after receipt of the written offer from PXP. Notwithstanding the foregoing, the restrictions and requirements in this section shall not apply (1) from and after the tenth anniversary of the Closing Date, (2) to opportunities not accepted by the Company in writing within thirty (30) days after receipt of a written offer from PXP, (3) from and after the time that PXP owns less than 50% of all the outstanding Common Stock on a Fully Diluted Basis, (4) to PXP’s investment in McMoRan Exploration Company or to any passive investments of PXP in which it holds not more than 10% of any outstanding class of capital stock (not involving board of directors representation) of a publicly held company that owns any AMI Assets, (5) to PXP’s acquisition of all of the stock or all the assets of any person that owns, among other things, any AMI Assets, as long as the fair market value of such person’s AMI Assets constitute less than 25% of the aggregate fair market value of the total assets being acquired by PXP pursuant to this clause (5) or (6) to any AMI Assets acquired by PXP after the date hereof which are subject to any area of mutual interest, buy-back, preferential or similar pre-emptive right to purchase in favor of third parties under the terms of Applicable Contracts; provided that PXP shall offer to the Company pursuant to this Section 2.02(c) any AMI Assets for which such third parties have failed to exercise their area of mutual interest, buy-back, preferential or similar pre-emptive rights to purchase and the time periods under this Section 2.02(c) shall be extended by the number of days such rights of third parties remain open. Within three days after any acquisition of any AMI Assets by PXP pursuant to clause (6) of the preceding sentence, PXP shall notify the Company (including the Preferred Nominee) and the Investors of such acquisition and the time periods for the exercise of such third party rights; provided, however, that PXP shall not be required to notify the Investors if it is contractually prohibited from providing such notification (but only to the extent of such prohibition). Notwithstanding the foregoing, in the event PXP acquires any AMI Assets and offers such AMI Assets to the Company in accordance with the first sentence of this Section 2.02(c), then to the extent the Company has available funds under the Approved Budget for the purchase of AMI Assets (up to an aggregate of $40 million), the Company shall be required to purchase such assets from PXP (and shall not be required to obtain Majority Preferred Approval for such purchase). The Company shall purchase such assets from PXP for an amount equal to the purchase price of such assets plus PXP’s reasonable transaction costs (without the requirement for the Common Stockholders, PXP or the Preferred Stockholders to contribute their proportionate share of the purchase price of such AMI Assets). If the Company accepts an offer from PXP to purchase any AMI Assets pursuant to this Section 2.02(c), then PXP and the Company will enter into an asset purchase agreement with the customary terms that are reasonably acceptable to each party; provided, however, it shall not be reasonable for the Company to refuse to execute an asset purchase agreement that contains identical or less burdensome terms than the terms by which PXP acquired the applicable AMI Assets.

4

3. If and only if Sections 1.01 and 2.02 of the Stockholders Agreement are amended pursuant to the terms of Section 2 of this Agreement, then Plains Exploration & Production Company shall pay to the Investors on the GOM Closing Date, in accordance with each Investors pro-rata share of Preferred Stock ownership, the total sum of $20,000,000.00 as an amendment fee, which, notwithstanding any provision of the Transaction Documents to the contrary, shall not be included in any determination of the Tax Adjustment Amount, and the Company may transfer on the GOM Closing Date all of its rights in the Winter Park Prospect, identified on Exhibit “A” attached hereto, to the seller(s) of the GOM Assets.

4. If the Company is not deemed to have waived any interest or right to purchase the GOM Assets and the Stockholders Agreement is not amended in accordance with Sections 1 and 2 of this Agreement, then nothing contained in this Agreement shall be deemed to be a waiver by the Company of any of its rights under Section 2.02(c) of the Stockholders Agreement with regard to any acquisitions by PXP of Deepwater GOM Assets. For the avoidance of doubt, nothing herein waives any rights other than those rights under Section 2.02(c).

5. Counterparts. This Agreement may be executed in counterparts, each of which shall be deemed an original instrument, but all such counterparts together shall constitute but one agreement. Facsimiles of signatures or signatures delivered in portable document format (.pdf) will be deemed to be originals.

6. Governing Law. This Agreement shall be governed and construed in accordance with the Laws of the State of Delaware, United States of America without regard to principles of conflicts of laws that would direct the application of the Laws of another jurisdiction

[Signature Pages Follow.]

5

IN WITNESS WHEREOF, this Agreement has been signed by each of the Parties as of the date first above written.

[Signature Page to Waiver Agreement]

[Signature Page to Waiver Agreement]

[Signature Page to Waiver Agreement]

[Signature Page to Waiver Agreement]

[Signature Page to Waiver Agreement]

[Signature Page to Waiver Agreement]

[Signature Page to Waiver Agreement]

Schedule B

Schedule B

Schedule B

EXHIBIT “A”

WINTER PARK PROSPECT