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v2.4.1.9

Condensed Consolidating Financial Statements - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract] |
Ownership percentage, Guarantors | 100.00%oas_OwnershipPercentageGuarantor

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

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- Details
Name: | us-gaap_CondensedFinancialInformationOfParentCompanyOnlyDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration