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Acquisitions - Additional Information (Details) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended	1 Months Ended	0 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 01, 2013 acre seller	Oct. 31, 2013	Sep. 26, 2013 acre
Business Acquisition [Line Items]
Revenues	$ 299,672,000us-gaap_OilAndGasRevenue	$ 368,659,000us-gaap_OilAndGasRevenue	$ 372,378,000us-gaap_OilAndGasRevenue	$ 349,519,000us-gaap_OilAndGasRevenue	$ 333,615,000us-gaap_OilAndGasRevenue	$ 305,498,000us-gaap_OilAndGasRevenue	$ 254,582,000us-gaap_OilAndGasRevenue	$ 248,304,000us-gaap_OilAndGasRevenue	$ 1,390,228,000us-gaap_OilAndGasRevenue	$ 1,141,999,000us-gaap_OilAndGasRevenue	$ 686,668,000us-gaap_OilAndGasRevenue
Operating income	15,924,000us-gaap_OperatingIncomeLoss	134,103,000us-gaap_OperatingIncomeLoss	166,545,000us-gaap_OperatingIncomeLoss	329,080,000us-gaap_OperatingIncomeLoss	122,133,000us-gaap_OperatingIncomeLoss	150,862,000us-gaap_OperatingIncomeLoss	113,450,000us-gaap_OperatingIncomeLoss	117,953,000us-gaap_OperatingIncomeLoss	645,652,000us-gaap_OperatingIncomeLoss	504,398,000us-gaap_OperatingIncomeLoss	276,993,000us-gaap_OperatingIncomeLoss
Capital spending commitment	4,800,000oas_CapitalSpendingCommitment								4,800,000oas_CapitalSpendingCommitment
West Williston Acquisition [Member]
Business Acquisition [Line Items]
Number of sellers												2oas_BusinessCombinationNumberofSellers / us-gaap_BusinessAcquisitionAxis = oas_WestWillistonAcquisitionMember
Acres acquired												136,000oas_BusinessCombinationAcresAcquired / us-gaap_BusinessAcquisitionAxis = oas_WestWillistonAcquisitionMember
Recognized identifiable assets acquired	1,475,361,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = oas_WestWillistonAcquisitionMember								1,475,361,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = oas_WestWillistonAcquisitionMember
Revenues													57,600,000us-gaap_OilAndGasRevenue / us-gaap_BusinessAcquisitionAxis = oas_WestWillistonAcquisitionMember
Operating income													14,900,000us-gaap_OperatingIncomeLoss / us-gaap_BusinessAcquisitionAxis = oas_WestWillistonAcquisitionMember
Acquisition costs													2,000,000us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_BusinessAcquisitionAxis = oas_WestWillistonAcquisitionMember
Consideration transferred									1,475,361,000us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = oas_WestWillistonAcquisitionMember
East Nesson [Member]
Business Acquisition [Line Items]
Acres acquired														25,000oas_BusinessCombinationAcresAcquired / us-gaap_BusinessAcquisitionAxis = oas_EastNessonMember
Recognized identifiable assets acquired	56,750,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = oas_EastNessonMember								56,750,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = oas_EastNessonMember
Consideration transferred									56,750,000us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = oas_EastNessonMember					56,800,000us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = oas_EastNessonMember
Capital spending commitment														$ 8,200,000oas_CapitalSpendingCommitment / us-gaap_BusinessAcquisitionAxis = oas_EastNessonMember

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- Definition

Business Combination, Acres Acquired

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This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

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Aggregate revenue during the period related to oil and gas business activities.

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The net result for the period of deducting operating expenses from operating revenues.

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