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Subsequent Events (Narrative) (Detail) (USD $) In Millions, unless otherwise specified		9 Months Ended
	Sep. 30, 2012 Maximum [Member] Amended Credit Facility [Member] EBITDAX [Member]	Sep. 30, 2012 Minimum [Member] Amended Credit Facility [Member] Current Ratio [Member]	Oct. 02, 2012 Subsequent Event [Member] Amended Credit Facility [Member]	Oct. 31, 2012 2012 Two-Way And Three-Way Collars [Member] Subsequent Event [Member] bbl	Oct. 31, 2012 2013 Two-Way And Three-Way Collars [Member] Subsequent Event [Member] bbl	Oct. 31, 2012 2014 Two-Way And Three-Way Collars [Member] Subsequent Event [Member] bbl
Subsequent Event [Line Items]
Increase in aggregate commitment			$ 750
Maximum debt to EBITDAX ratio	4
Minimum current ratio		1.0
Maximum EBITDAX to Interest Expense ratio			2.5
Total notional amount, in barrels				668,000	396,000	31,000

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

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

X

- Definition

The maximum ratio of Total Net Debt to consolidated EBITDAX as required by the entity's debt agreement.

+ References

No definition available.

+ Details

Name:	oas_MaximumDebtToEbitdaxRatio
Namespace Prefix:	oas_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The minimum current ratio as required by the entity's debt agreement.

+ References

No definition available.

+ Details

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Namespace Prefix:	oas_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The maximum ratio of consolidated EBITDAX to consolidated Interest Expense as required by the entity's debt agreement.

+ References

No definition available.

+ Details

Name:	oas_RatioOfEbitdaxToInterestExpenseNecessaryToMaintainCovenants
Namespace Prefix:	oas_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration