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Common Stock - Additional Information (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 09, 2013
Class of Stock [Line Items]        
Common stock, par value (in usd per share) $ 0.01us-gaap_CommonStockParOrStatedValuePerShare $ 0.01us-gaap_CommonStockParOrStatedValuePerShare    
Shares issued, price per share       $ 44.94us-gaap_SharesIssuedPricePerShare
Proceeds from issuance of common stock   $ 314,400oas_ProceedsfromIssuanceofCommonStockNetofDiscountsandExpenses    
Principal payments on revolving credit facility 455,570us-gaap_RepaymentsOfLongTermLinesOfCredit 264,430us-gaap_RepaymentsOfLongTermLinesOfCredit 0us-gaap_RepaymentsOfLongTermLinesOfCredit  
Common Stock [Member]        
Class of Stock [Line Items]        
Issuance of common stock, shares       7,000,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
Common stock, par value (in usd per share)       $ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
Common Stock [Member]        
Class of Stock [Line Items]        
Issuance of common stock, shares   7,000,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
   
Proceeds from issuance of common stock   70oas_ProceedsfromIssuanceofCommonStockNetofDiscountsandExpenses
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
   
Additional Paid-in Capital [Member]        
Class of Stock [Line Items]        
Proceeds from issuance of common stock   314,300oas_ProceedsfromIssuanceofCommonStockNetofDiscountsandExpenses
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
   
Revolving Credit Facility [Member]        
Class of Stock [Line Items]        
Principal payments on revolving credit facility   $ 264,400us-gaap_RepaymentsOfLongTermLinesOfCredit
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember