Condensed Consolidating Financial Information (Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Ownership percentage, guarantor			100.00%
Parent/Issuer [Member]
Equity in earnings from subsidiaries	$ 32,735	$ 71,445	$ 145,021	$ 105,832

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

No definition available.

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