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Consolidating Financial Statements (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Change in Issuer's Investment		$ 54,400,000
Parent/Issuer [Member]
Equity In Earnings In Subsidiaries	$ 99,836,000	$ (28,124,000)

X

- Definition

Change in Issuer's Investments in and advances to subsidiaries.

+ References

No definition available.

+ Details

Name:	oas_ChangeInIssuersInvestment
Namespace Prefix:	oas_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its consolidated subsidiaries.

+ References

No definition available.

+ Details

Name:	oas_EquityInEarningsInSubsidiaries
Namespace Prefix:	oas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration