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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

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

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- 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