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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