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Supplemental Oil and Gas Reserve Information - Unaudited - Additional Information (Detail) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2014 MBoe | Dec. 31, 2014 MBbls MBoe | Dec. 31, 2013 MBbls | Dec. 31, 2012 MBoe MMcf
Reserve Quantities [Line Items] |  |  |  |
Proved Developed and Undeveloped Reserves, Purchases of Minerals in Place (Energy) |  | 1,799 us-gaap_ProvedDevelopedAndUndevelopedReservePurchaseOfMineralInPlaceEnergy | 45,731 us-gaap_ProvedDevelopedAndUndevelopedReservePurchaseOfMineralInPlaceEnergy | 966 us-gaap_ProvedDevelopedAndUndevelopedReservePurchaseOfMineralInPlaceEnergy
Proved Developed and Undeveloped Reserves, Extensions, Discoveries, and Additions (Energy) |  | 92,596 us-gaap_ProvedDevelopedAndUndevelopedReserveExtensionAndDiscoveryEnergy | 46,094 us-gaap_ProvedDevelopedAndUndevelopedReserveExtensionAndDiscoveryEnergy | 73,871 us-gaap_ProvedDevelopedAndUndevelopedReserveExtensionAndDiscoveryEnergy
Estimated extensions and discoveries of reserves associated with new wells |  | 34,404 oas_EstimatedExtensionsAndDiscoveriesOfReservesAssociatedWithNewWells | 22,190 oas_EstimatedExtensionsAndDiscoveriesOfReservesAssociatedWithNewWells | 16,548 oas_EstimatedExtensionsAndDiscoveriesOfReservesAssociatedWithNewWells
Additional proved undeveloped reserves |  | 58,192 oas_AdditionalProvedUndevelopedReserves | 23,904 oas_AdditionalProvedUndevelopedReserves | 57,323 oas_AdditionalProvedUndevelopedReserves
Percentage of reserves produced from wells in certain formations |  | 100.00%oas_PercentageOfReservesProducedFromWells | 100.00%oas_PercentageOfReservesProducedFromWells | 100.00%oas_PercentageOfReservesProducedFromWells
Percentage of proved undeveloped reserves |  | 100.00%oas_PercentageOfProvedUndevelopedReserves | 100.00%oas_PercentageOfProvedUndevelopedReserves | 100.00%oas_PercentageOfProvedUndevelopedReserves
Proved Developed and Undeveloped Reserves, Sales of Minerals in Place (Energy) | (8,448)us-gaap_ProvedDevelopedAndUndevelopedReservesSaleOfMineralInPlaceEnergy | (8,448)us-gaap_ProvedDevelopedAndUndevelopedReservesSaleOfMineralInPlaceEnergy | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesSaleOfMineralInPlaceEnergy | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesSaleOfMineralInPlaceEnergy
Reserves divested associated with non-operated properties sold |  | 7,640 us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace | 0 us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace
Net positive (negative) revisions of previous estimates |  | (25,117)us-gaap_ProvedDevelopedAndUndevelopedReserveRevisionOfPreviousEstimateEnergy | 5,125 us-gaap_ProvedDevelopedAndUndevelopedReserveRevisionOfPreviousEstimateEnergy | (1,983)us-gaap_ProvedDevelopedAndUndevelopedReserveRevisionOfPreviousEstimateEnergy
Percentage of net positive (negative) revisions |  | (11.00%)oas_RevisionsPercentage | 3.60%oas_RevisionsPercentage | (2.50%)oas_RevisionsPercentage
Proved Undeveloped Reserves, Number of Locations |  | 80 oas_ProvedUndevelopedReservesNumberofLocations |  |
Proved Undeveloped Reserves, Number of Locations, Net |  | 56.2 oas_ProvedUndevelopedReservesNumberofLocationsNet |  |
Proved Undeveloped Reserves, Decrease from Estimated Net Proved Reserve Balance (Energy) |  | 21,411 oas_ProvedUndevelopedReservesDecreasefromEstimatedNetProvedReserveBalanceEnergy |  |
Discount rate of future cash flows |  | 10.00%oas_DiscountRateOfFutureCashFlows |  |
Unweighted arithmetic average first-day-of-the-month prices for oil, price per Bbl |  | 95.28 oas_UnweightedArithmeticAverageFirstDayOfMonthPrices | 96.96 oas_UnweightedArithmeticAverageFirstDayOfMonthPrices | 94.68 oas_UnweightedArithmeticAverageFirstDayOfMonthPrices
Unweighted arithmetic average first-day-of-the-month prices for natural gas, price per MMBtu |  | 4.35 oas_UnweightedArithmeticAverageFirstDayOfMonthPricesForNaturalGas | 3.66 oas_UnweightedArithmeticAverageFirstDayOfMonthPricesForNaturalGas | 2.75 oas_UnweightedArithmeticAverageFirstDayOfMonthPricesForNaturalGas
Burke and Mountrail counties [Member] |  |  |  |
Reserve Quantities [Line Items] |  |  |  |
Proved Developed and Undeveloped Reserves, Purchases of Minerals in Place (Energy) |  |  |  | 966 us-gaap_ProvedDevelopedAndUndevelopedReservePurchaseOfMineralInPlaceEnergy / us-gaap_StatementGeographicalAxis = oas_BurkeAndMountrailCountyMember

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