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
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Standardized Measure of Discounted Future Net Cash Flows
January 1	$ 3,727,559	$ 2,259,897	$ 1,319,471
Net changes in prices and production costs	(588,212)	254,979	(7,814)
Net changes in future development costs	(61,760)	57,566	28,124
Sales of oil and natural gas, net	(979,938)	(857,540)	(542,515)
Proved oil and gas reserves increase	0	0	0
Purchases of reserves in place	38,035	858,382	15,890
Sales of reserves in place	(251,002)	0	0
Revisions of previous quantity estimates	(604,651)	99,954	(47,957)
Previously estimated development costs incurred	249,926	373,912	480,925
Accretion of discount	548,690us-gaap_AccretionOfDiscount	346,068us-gaap_AccretionOfDiscount	190,370us-gaap_AccretionOfDiscount
Net change in income taxes	259,592	(774,910)	(400,196)
Changes in timing and other	(107,585)	(1,951)	(134,880)
December 31	3,981,661	3,727,559	2,259,897
Extensions [Member]
Standardized Measure of Discounted Future Net Cash Flows
Proved oil and gas reserves increase	$ 1,751,007	$ 1,111,202	$ 1,358,479

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

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Standardized Measure of Discounted Future Net Cash Flows

January 1

$ 3,727,559us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves

$ 2,259,897us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves

$ 1,319,471us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves

Net changes in prices and production costs

(588,212)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts

254,979us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts

(7,814)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts

Net changes in future development costs

(61,760)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts

57,566us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts

28,124us-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

0us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts

Purchases of reserves in place

38,035us-gaap_IncreaseDueToPurchasesOfMineralsInPlace

858,382us-gaap_IncreaseDueToPurchasesOfMineralsInPlace

15,890us-gaap_IncreaseDueToPurchasesOfMineralsInPlace

Sales of reserves in place

(251,002)us-gaap_DecreaseDueToSalesOfMineralsInPlace

0us-gaap_DecreaseDueToSalesOfMineralsInPlace

Revisions of previous quantity estimates

(604,651)us-gaap_RevisionsOfPreviousQuantityEstimates

99,954us-gaap_RevisionsOfPreviousQuantityEstimates

(47,957)us-gaap_RevisionsOfPreviousQuantityEstimates

Previously estimated development costs incurred

249,926us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod

373,912us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod

480,925us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod

Accretion of discount

548,690us-gaap_AccretionOfDiscount

346,068us-gaap_AccretionOfDiscount

190,370us-gaap_AccretionOfDiscount

Net change in income taxes

259,592us-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

3,981,661us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves

3,727,559us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves

2,259,897us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves

Extensions [Member]

Standardized Measure of Discounted Future Net Cash Flows

Proved oil and gas reserves increase

$ 1,751,007us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts
/ us-gaap_AverageSalesPriceAndProductionCostsPerUnitOfProductionProductionTypeAxis
= oas_ExtensionsMember

$ 1,111,202us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts
/ us-gaap_AverageSalesPriceAndProductionCostsPerUnitOfProductionProductionTypeAxis
= oas_ExtensionsMember

$ 1,358,479us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts
/ us-gaap_AverageSalesPriceAndProductionCostsPerUnitOfProductionProductionTypeAxis
= oas_ExtensionsMember

- Definition

Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of passage of time.

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