UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): September 24, 2013

OASIS PETROLEUM INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (281) 404-9500

Not Applicable.

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Item 1.01 Entry into a Material Definitive Agreement.

Supplemental Indenture for 6.875% Notes due 2022

On September 24, 2013, Oasis Petroleum Inc. (the “Company”) completed its offering of $1.0 billion in aggregate principal amount of its 6.875% senior unsecured notes due 2022 (the “Notes”), which are fully and unconditionally guaranteed on a senior unsecured basis by certain of the Company’s subsidiaries (collectively, the “Guarantors”): Oasis Petroleum LLC, Oasis Petroleum North America LLC, Oasis Petroleum Marketing LLC, Oasis Well Services LLC and Oasis Midstream Services LLC.

The terms of the Notes are governed by the indenture dated as of November 10, 2011 (the “Base Indenture”), among the Company and U.S. Bank, National Association, as trustee (the “Trustee”), as amended and supplemented by the fourth supplemental indenture, dated as of September 24, 2013, among the Company, the Guarantors and the Trustee (the “Supplemental Indenture”; the Base Indenture, as amended and supplemented, the “Indenture”).

The Notes will mature on March 15, 2022, and interest is payable on the Notes on each March 15 and September 15, commencing on March 15, 2014. At any time prior to September 15, 2016, the Company may redeem up to 35% of the Notes at a redemption price of 106.875% of the principal amount, plus accrued and unpaid interest to the redemption date, with the proceeds of certain equity offerings so long as the redemption occurs within 180 days of completing such equity offering and at least 65% of the aggregate principal amount of the Notes remains outstanding after such redemption. Prior to September 15, 2017, the Company may redeem some or all of the notes for cash at a redemption price equal to 100% of their principal amount plus an applicable make-whole premium and accrued and unpaid interest to the redemption date. If the Company undergoes a change of control on or prior to September 15, 2014, the Company may redeem all, but not less than all, of the Notes at a redemption price equal to 110% of the principal amount of the Notes redeemed plus accrued and unpaid interest. The Company must offer to purchase the Notes if it experiences specific kinds of changes of control or sells assets under certain circumstances. On and after September 15, 2017, the Company may redeem some or all of the notes at redemption prices (expressed as percentages of principal amount) equal to 103.438% for the twelve-month period beginning on September 15, 2017, 101.719% for the twelve-month period beginning September 15, 2018, and 100.000% beginning on September 15, 2019, plus accrued and unpaid interest to the redemption date.

The Notes are the Company’s senior unsecured obligations, and will rank equally in right of payment with all of the Company’s existing and future senior debt, and will rank senior in right of payment to all of the Company’s future subordinated debt. The Notes will be effectively subordinated to all of the Company’s existing and future secured debt to the extent of the value of the collateral securing such indebtedness.

The Indenture restricts the Company’s ability and the ability of certain of its subsidiaries to: (i) incur additional debt or enter into sale and leaseback transactions; (ii) pay distributions on, redeem or repurchase, equity interests; (iii) make certain investments; (iv) incur liens; (v) enter into transactions with affiliates; (vi) merge or consolidate with another company; and (vii) transfer and sell assets. These covenants are subject to a number of important exceptions and qualifications. If at any time when the Notes are rated investment grade by both Moody’s Investors Service, Inc. and Standard & Poor’s Ratings Services and no Default (as defined in the Indenture) has occurred and is continuing, many of such covenants will terminate and the Company and its subsidiaries will cease to be subject to such covenants.

The Indenture contains customary events of default, including:

A copy of the Supplemental Indenture is filed as Exhibit 4.1 to this Form 8-K and is incorporated herein by reference. The description of the Indenture and Supplemental Indenture are summaries and are qualified in their entirety by the terms of the Indenture and the Supplemental Indenture.

Registration Rights Agreement

In connection with the issuance and sale of the Notes, the Company and the Guarantors entered into a registration rights agreement (the “Registration Rights Agreement”) with the Initial Purchasers, dated September 24, 2013. Pursuant to the Registration Rights Agreement, the Company and the Guarantors have agreed to file a registration statement with the Securities and Exchange Commission so that holders of the Notes can exchange the Notes for registered notes (the “Exchange Notes”) that have substantially identical terms as the Notes. In addition, the Company and the Guarantors have agreed to exchange the guarantee related to the Notes for a registered guarantee having substantially the same terms as the original guarantee. The Company and the Guarantors will use commercially reasonable efforts to cause the exchange to be completed within 360 days after the issuance of the Notes. The Company and the Guarantors are required to pay additional interest if they fail to comply with their obligations to register the Notes within the specified time periods.

A copy of the Registration Rights Agreement is filed as Exhibit 4.2 to this Form 8-K and is incorporated herein by reference. The description of the Registration Rights Agreement in this report is a summary and is qualified in its entirety by the terms of the Registration Rights Agreement.

Item 2.03 Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant.

The information included in Item 1.01 of this Current Report on Form 8-K is incorporated by reference into this Item 2.03 of this Current Report on Form 8-K.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

EXHIBIT INDEX