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Income Taxes - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Contingency [Line Items]      
U.S. federal tax statutory rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Net operating loss $ 30,200,000us-gaap_OperatingLossCarryforwards    
Current income tax 134,000us-gaap_CurrentIncomeTaxExpenseBenefit 475,000us-gaap_CurrentIncomeTaxExpenseBenefit 7,000us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred tax asset related to net operating loss carryforwards 35,650,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards 17,215,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards  
Unrealized excess tax benefits from stock-based compensation 3,500,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost    
Unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefits    
Deferred tax liability, current 97,499,000us-gaap_DeferredTaxLiabilitiesCurrent 0us-gaap_DeferredTaxLiabilitiesCurrent  
Deferred income tax asset, current   6,335,000us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent  
Domestic Tax Authority [Member]      
Income Tax Contingency [Line Items]      
Net operating loss 104,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		    
Stock-based compensation related to unrealized excess tax benefits 9,500,000oas_StockbasedCompensationRelatedToUnrealizedExcessTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		    
State and Local Jurisdiction [Member]      
Income Tax Contingency [Line Items]      
Net operating loss 77,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		    
Stock-based compensation related to unrealized excess tax benefits $ 7,400,000oas_StockbasedCompensationRelatedToUnrealizedExcessTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember