EXHIBIT 5.1

                                                                  March 1, 2002



                         [Baker Botts L.L.P. Letterhead]


Marathon Oil Corporation
5555 San Felipe Road
Houston, Texas 77056-2799

Gentlemen:

         Marathon Oil Corporation, a Delaware corporation ("Marathon"), has
engaged us to render the opinions we express below in connection with its
offering of $450,000,000 aggregate principal amount of notes due March 15, 2012
(the "2012 Notes") and $550,000,000 aggregate principal amount of notes due
March 15, 2032 (the "2032 Notes" and, together with the 2012 Notes, the
"Notes"), which it is offering under the registration statement on Form S-3, as
amended by post-effective amendment no. 2 thereto (Reg. No. 333-88797), it filed
with the Securities and Exchange Commission (the "SEC") under the Securities Act
of 1933 (the "1933 Act"), which relates to Marathon's offering and sale of
various securities under the 1933 Act's Rule 415. In this letter, "Registration
Statement" means that registration statement, as amended by that post-effective
amendment when it became effective under the 1933 Act.

         The Company will issue the Notes under an indenture dated as of
February 26, 2002 between it and JPMorgan Chase Bank, as trustee, relating to
senior debt securities of Marathon (the "Indenture").

                  For purposes of the opinions we express below, we have
         examined, among other agreements, instruments and documents, the
         following:

                  o the restated certificate of incorporation and by-laws, each
         as amended to date, of Marathon;

                  o the Registration Statement and its exhibits;

                  o the prospectus the Registration Statement includes;

                  o the prospectus supplement dated February 27, 2002 that
         Marathon prepared and filed with the SEC on February 28, 2002 under the
         1933 Act's Rule 424(b)(5);

                  o the Indenture;

                  o the underwriting agreement and related pricing agreement,
         each dated February 27, 2002 among Marathon and Banc of America
         Securities LLC and Salomon Smith Barney, as representatives of the
         several underwriters, relating to the offering and sale of the Notes
         (the "Underwriting Agreements");

Marathon Oil Company                    -2-                      March 1, 2002


                  o the resolutions of the board of directors of Marathon (the
         "Board") and a committee of its officers the Board has established
         which authorize the issuance of the Notes and approve the terms of the
         offering and sale of the Notes, including the Establishment Action (as
         defined in the Indenture) relating to the Notes;

                  o corporate records of Marathon, including minute books it has
         furnished us; and

                  o certificates of public officials and of representatives of
         Marathon.

In giving these opinions, we have relied on certificates of officers of Marathon
and of public officials with respect to the accuracy of the factual matters
those certificates cover or contain, and we have assumed that all signatures on
documents we have examined are genuine, all documents submitted to us as
originals are authentic, all documents submitted to us as certified or
photostatic copies conform to the original copies of those documents and those
original copies are authentic.

         On the basis of and subject to the foregoing, we are of the opinion
that:

                  1. Marathon is a corporation duly incorporated and validly
         existing in good standing under the laws of the State of Delaware.

                  2. The Notes will, when they have been duly executed,
         authenticated, issued and delivered in accordance with the provisions
         of the Indenture and the Establishment Action, and duly purchased and
         paid for in accordance with the provisions of the Underwriting
         Agreements, constitute legal, valid and binding obligations of
         Marathon, enforceable against Marathon in accordance with their terms,
         except as that enforcement is subject to (a) any applicable bankruptcy,
         insolvency, reorganization, moratorium, fraudulent transfer or
         conveyance or other laws relating to or affecting creditors' rights
         generally, (b) general principles of equity, regardless of whether that
         enforceability is considered in a proceeding in equity or at law, and
         (c) any implied covenants of good faith and fair dealing.

         We limit the opinions we express above in all respects to matters of
the laws of the State of New York and the General Corporation Law of the State
of Delaware, each as in effect on the date hereof.

         We hereby consent to the filing of this opinion of counsel as Exhibit
5.1 to Marathon's current report on Form 8-K reporting the offering of the
Notes. In giving this consent, we do not hereby admit we are in the category of
persons whose written consent Section 7 of the 1933 Act requires to be filed
with the Registration Statement.

                                       Very truly yours,


                                       /s/ BAKER BOTTS L.L.P.