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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