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Subsequent Events - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 Senior secured revolving line of credit [Member]	Oct. 02, 2012 Amended Credit Facility [Member]	Apr. 05, 2013 Subsequent Event [Member] Senior secured revolving line of credit [Member]	Apr. 05, 2013 Subsequent Event [Member] Amended Credit Facility [Member]	Apr. 30, 2013 2013 swaps, sub-floors and two-way costless collar options [Member] Subsequent Event [Member] Barrels	Apr. 30, 2013 2014 swaps, sub-floors and two-way costless collar options [Member] Subsequent Event [Member] Barrels	Apr. 30, 2013 2015 swaps, sub-floors and two-way costless collar options [Member] Subsequent Event [Member] Barrels
Subsequent Event [Line Items]
Letter of credit maximum borrowings		$ 750		$ 1,250
Letter of credit borrowings	$ 1,000	$ 500	$ 2,500	$ 900
Total notional amount of oil barrels					321,000	1,549,500	139,500

X

- Definition

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

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Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The aggregate commitment may be increased to this amount by increasing the commitment of one or more lender(s) under the credit facility.

+ References

No definition available.

+ Details

Name:	oas_LineOfCreditFacilityMaximumIncreaseToAggregateCommitments
Namespace Prefix:	oas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration