<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-01-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000101778</issuerCik>
        <issuerName>MARATHON OIL CORP</issuerName>
        <issuerTradingSymbol>MRO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001237640</rptOwnerCik>
            <rptOwnerName>LADER PHILIP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MARATHON OIL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>5555 SAN FELIPE ROAD</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77056</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Marathon Oil Corporation Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-01-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>9853.604</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>121857.028</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Marathon Oil Corporation Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-01-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>0.457</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>121856.571</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Marathon Oil Corporation Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3277.501</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Trust</value>
                    <footnoteId id="F5"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Non-Retainer Annual Director Stock Unit Award granted under Marathon Oil Corporation 2016 Incentive Compensation Plan. These common stock units, along with their accrued dividend equivalent rights, are payable in shares of common stock upon the reporting person's departure from the Board of Directors.</footnote>
        <footnote id="F2">Includes dividends of 1531.991 shares previously not reported pursuant to Rule 16a-11.</footnote>
        <footnote id="F3">Fractional stock units remaining upon vesting of common stock units on January 3, 2017 are forfeited pursuant to the terms of the Directors' Deferred Comp Plan.</footnote>
        <footnote id="F4">Includes dividends of 46.838 shares previously not reported pursuant to Rule 16a-11.</footnote>
        <footnote id="F5">The shares are held by the reporting person's spouse's irrevocable defective grantor trust (&quot;Spouse's Trust&quot;). The reporting person is the trustee of the Spouse's Trust and the reporting person's spouse is the settlor of the Spouse's Trust. The beneficiaries under the Spouse's Trust are the children of the reporting person and spouse.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Kim Warnica, Attorney-in-Fact for Philip Lader</signatureName>
        <signatureDate>2017-01-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2017-01-03
0
0000101778
MARATHON OIL CORP
MRO
0001237640
LADER PHILIP
C/O MARATHON OIL CORPORATION
5555 SAN FELIPE ROAD
HOUSTON
TX
77056
1
0
0
0
Marathon Oil Corporation Common Stock
2017-01-03
4
A
0
9853.604
0
A
121857.028
D
Marathon Oil Corporation Common Stock
2017-01-03
4
D
0
0.457
0
D
121856.571
D
Marathon Oil Corporation Common Stock
3277.501
I
Trust
Non-Retainer Annual Director Stock Unit Award granted under Marathon Oil Corporation 2016 Incentive Compensation Plan. These common stock units, along with their accrued dividend equivalent rights, are payable in shares of common stock upon the reporting person's departure from the Board of Directors.
Includes dividends of 1531.991 shares previously not reported pursuant to Rule 16a-11.
Fractional stock units remaining upon vesting of common stock units on January 3, 2017 are forfeited pursuant to the terms of the Directors' Deferred Comp Plan.
Includes dividends of 46.838 shares previously not reported pursuant to Rule 16a-11.
The shares are held by the reporting person's spouse's irrevocable defective grantor trust ("Spouse's Trust"). The reporting person is the trustee of the Spouse's Trust and the reporting person's spouse is the settlor of the Spouse's Trust. The beneficiaries under the Spouse's Trust are the children of the reporting person and spouse.
Kim Warnica, Attorney-in-Fact for Philip Lader
2017-01-04