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Condensed Consolidating Financial Information (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Ownership percentage, guarantor	100.00%
Equity in earnings from subsidiaries
Parent/Issuer [Member]
Equity in earnings from subsidiaries	$ 26,264	$ (3,170)

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

X

- Definition

Ownership percentage, guarantor

+ References

No definition available.

+ Details

Name:	oas_OwnershipPercentageGuarantor
Namespace Prefix:	oas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration