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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-01-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001181653</rptOwnerCik>
            <rptOwnerName>DABERKO DAVID A</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>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON STOCK</value>
            </securityTitle>
            <transactionDate>
                <value>2006-01-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1916.002</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>65.24</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12342.939</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Annual Director Stock Awards granted under Marathon Oil Corporation 2003 Incentive Compensation Plan. The annual non-retainer stock-based compensation represented by these common stock units vest upon reporting person's retirement from the Board of Directors. Shares of common stock will be issued upon vesting of common stock units.</footnote>
        <footnote id="F2">Includes dividends of 181.075 shares previously not reported pursuant to Rule 16a-11.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>R. J. Kolencik, Attorney-in-Fact for David A. Daberko</signatureName>
        <signatureDate>2006-01-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2006-01-03
0
0000101778
MARATHON OIL CORP
MRO
0001181653
DABERKO DAVID A
C/O MARATHON OIL CORPORATION
5555 SAN FELIPE ROAD
HOUSTON
TX
77056
1
0
0
0
COMMON STOCK
2006-01-03
4
A
0
1916.002
65.24
A
12342.939
D
Annual Director Stock Awards granted under Marathon Oil Corporation 2003 Incentive Compensation Plan. The annual non-retainer stock-based compensation represented by these common stock units vest upon reporting person's retirement from the Board of Directors. Shares of common stock will be issued upon vesting of common stock units.
Includes dividends of 181.075 shares previously not reported pursuant to Rule 16a-11.
R. J. Kolencik, Attorney-in-Fact for David A. Daberko
2006-01-05