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Supplemental Oil and Gas Reserve Information - Unaudited - Changes in Standardized Measure of Discounted Future Net Cash Flows Applicable to Proved Oil and Natural Gas Reserves (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Standardized Measure of Discounted Future Net Cash Flows |  |  |
January 1 | $ 3,727,559 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 2,259,897 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 1,319,471 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves
Net changes in prices and production costs | (588,212)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts | 254,979 us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts | (7,814)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts
Net changes in future development costs | (61,760)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts | 57,566 us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts | 28,124 us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts
Sales of oil and natural gas, net | (979,938)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts | (857,540)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts | (542,515)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts
Proved oil and gas reserves increase | 0 us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts | 0 us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts | 0 us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts
Purchases of reserves in place | 38,035 us-gaap_IncreaseDueToPurchasesOfMineralsInPlace | 858,382 us-gaap_IncreaseDueToPurchasesOfMineralsInPlace | 15,890 us-gaap_IncreaseDueToPurchasesOfMineralsInPlace
Sales of reserves in place | (251,002)us-gaap_DecreaseDueToSalesOfMineralsInPlace | 0 us-gaap_DecreaseDueToSalesOfMineralsInPlace | 0 us-gaap_DecreaseDueToSalesOfMineralsInPlace
Revisions of previous quantity estimates | (604,651)us-gaap_RevisionsOfPreviousQuantityEstimates | 99,954 us-gaap_RevisionsOfPreviousQuantityEstimates | (47,957)us-gaap_RevisionsOfPreviousQuantityEstimates
Previously estimated development costs incurred | 249,926 us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod | 373,912 us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod | 480,925 us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod
Accretion of discount | 548,690 us-gaap_AccretionOfDiscount | 346,068 us-gaap_AccretionOfDiscount | 190,370 us-gaap_AccretionOfDiscount
Net change in income taxes | 259,592 us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves | (774,910)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves | (400,196)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves
Changes in timing and other | (107,585)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther | (1,951)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther | (134,880)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther
December 31 | 3,981,661 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | 3,727,559 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | 2,259,897 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves
Extensions [Member] |  |  |
Standardized Measure of Discounted Future Net Cash Flows |  |  |
Proved oil and gas reserves increase | $ 1,751,007 us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts / us-gaap_AverageSalesPriceAndProductionCostsPerUnitOfProductionProductionTypeAxis = oas_ExtensionsMember | $ 1,111,202 us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts / us-gaap_AverageSalesPriceAndProductionCostsPerUnitOfProductionProductionTypeAxis = oas_ExtensionsMember | $ 1,358,479 us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts / us-gaap_AverageSalesPriceAndProductionCostsPerUnitOfProductionProductionTypeAxis = oas_ExtensionsMember

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