Condensed Consolidating Financial Statements - Schedule of Consolidating Statement of Cash Flows (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$ 506,877,000us-gaap_NetIncomeLoss	$ 227,959,000us-gaap_NetIncomeLoss	$ 153,388,000us-gaap_NetIncomeLoss
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation, depletion and amortization	412,334,000	307,055,000	206,734,000
Gain (loss) on sale of properties	(186,999,000)us-gaap_GainLossOnSaleOfProperty	0us-gaap_GainLossOnSaleOfProperty	0us-gaap_GainLossOnSaleOfProperty
Impairment of oil and gas properties	47,238,000us-gaap_ImpairmentOfOilAndGasProperties	1,168,000us-gaap_ImpairmentOfOilAndGasProperties	3,581,000us-gaap_ImpairmentOfOilAndGasProperties
Deferred income taxes	307,457,000us-gaap_DeferredIncomeTaxExpenseBenefit	134,583,000us-gaap_DeferredIncomeTaxExpenseBenefit	92,479,000us-gaap_DeferredIncomeTaxExpenseBenefit
Derivative, Gain (Loss) on Derivative, Net	(327,011,000)	35,432,000	(34,164,000)
Stock-based compensation expenses	21,302,000us-gaap_ShareBasedCompensation	11,982,000us-gaap_ShareBasedCompensation	10,333,000us-gaap_ShareBasedCompensation
Deferred financing costs amortization and other	11,028,000oas_DebtDiscountAmortizationAndOther	4,248,000oas_DebtDiscountAmortizationAndOther	2,810,000oas_DebtDiscountAmortizationAndOther
Working capital and other changes:
Change in accounts receivable	16,702,000	(107,473,000)	(90,103,000)
Change in inventory	(3,776,000)us-gaap_IncreaseDecreaseInInventories	(13,941,000)us-gaap_IncreaseDecreaseInInventories	(29,313,000)us-gaap_IncreaseDecreaseInInventories
Change in prepaid expenses	(3,199,000)	(8,191,000)	346,000
Change in other current assets	(6,135,000)	(56,000)	156,000
Change in other assets	114,000	(3,248,000)	(95,000)
Change in accounts payable and accrued liabilities	76,723,000	107,451,000	76,706,000
Change in other current liabilities	0	0	(472,000)
Change in other liabilities	(139,000)	887,000	0
Net cash provided by operating activities	872,516,000	697,856,000	392,386,000
Cash flows from investing activities:
Payments to Explore and Develop Oil and Gas Properties	(1,354,281,000)	(893,524,000)	(1,051,365,000)
Acquisition of oil and gas properties	(46,247,000)	(1,560,072,000)	0
Proceeds from sale of properties	324,852,000
Costs related to sale of properties	(2,337,000)	0	0
Derivative settlements	6,774,000	(8,133,000)	6,545,000
Purchases of short-term investments	0	0	(126,213,000)
Redemptions of short-term investments	0	25,000,000	120,316,000
Advances from joint interest partners	(6,213,000)	(8,347,000)	12,112,000
Net cash used in investing activities	(1,077,452,000)	(2,445,076,000)	(1,038,605,000)
Cash flows from financing activities:
Proceeds from issuance of senior notes	0	1,000,000,000	400,000,000
Proceeds from revolving credit facility	620,000,000	600,000,000	0
Principal payments on revolving credit facility	(455,570,000)	(264,430,000)	0
Debt issuance costs	(99,000)us-gaap_PaymentsOfDebtIssuanceCosts	(22,910,000)us-gaap_PaymentsOfDebtIssuanceCosts	(8,012,000)us-gaap_PaymentsOfDebtIssuanceCosts
Proceeds from sale of common stock	0	314,580,000	0
Purchases of treasury stock	(5,309,000)	(1,566,000)	(3,194,000)
Proceeds from (Payments for) Other Financing Activities	(176,000)	0	0
Net cash provided by financing activities	158,846,000	1,625,674,000	388,794,000
Decrease in cash and cash equivalents	(46,090,000)	(121,546,000)	(257,425,000)
Beginning of period	91,901,000	213,447,000	470,872,000
End of period	45,811,000	91,901,000	213,447,000
Parent/Issuer [Member]
Cash flows from operating activities:
Net income	506,877,000	227,959,000	153,388,000
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in earnings of subsidiaries	(613,601,000)	(299,459,000)	(202,924,000)
Deferred income taxes	(64,029,000)	(42,542,000)	(29,588,000)
Stock-based compensation expenses	20,701,000	11,602,000	10,219,000
Deferred financing costs amortization and other	4,549,000	4,018,000	2,277,000
Working capital and other changes:
Change in accounts receivable	(11,000)	(460,000)	(222,000)
Change in prepaid expenses	21,000	(5,000)	(4,000)
Change in other current assets	0	235,000	(217,000)
Change in other assets	0	0	25,000
Change in accounts payable and accrued liabilities	84,044,000	20,710,000	18,612,000
Net cash provided by operating activities	(61,449,000)	(77,942,000)	(48,434,000)
Cash flows from investing activities:
Purchases of short-term investments			(126,213,000)
Redemptions of short-term investments		25,000,000	120,316,000
Net cash used in investing activities	0	25,000,000	(5,897,000)
Cash flows from financing activities:
Proceeds from issuance of senior notes		1,000,000,000	400,000,000
Debt issuance costs	0	(16,362,000)	(7,307,000)
Proceeds from sale of common stock		314,580,000
Purchases of treasury stock	(5,309,000)	(1,566,000)	(3,194,000)
Investment in / capital contributions from subsidiaries	33,433,000	(1,343,230,000)	(644,853,000)
Proceeds from (Payments for) Other Financing Activities	(176,000)
Net cash provided by financing activities	27,948,000	(46,578,000)	(255,354,000)
Decrease in cash and cash equivalents	(33,501,000)	(99,520,000)	(309,685,000)
Beginning of period	34,277,000	133,797,000	443,482,000
End of period	776,000	34,277,000	133,797,000
Combined Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net income	613,601,000	299,459,000	202,924,000
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation, depletion and amortization	412,334,000	307,055,000	206,734,000
Gain (loss) on sale of properties	(186,999,000)
Impairment of oil and gas properties	47,238,000	1,168,000	3,581,000
Deferred income taxes	371,486,000	177,125,000	122,067,000
Derivative, Gain (Loss) on Derivative, Net	(327,011,000)	35,432,000	(34,164,000)
Stock-based compensation expenses	601,000	380,000	114,000
Deferred financing costs amortization and other	6,479,000	230,000	533,000
Working capital and other changes:
Change in accounts receivable	(65,657,000)	(110,266,000)	(94,106,000)
Change in inventory	(3,776,000)	(13,941,000)	(29,313,000)
Change in prepaid expenses	(3,220,000)	(8,186,000)	350,000
Change in other current assets	(6,135,000)	(291,000)	373,000
Change in other assets	114,000	(3,248,000)	(120,000)
Change in accounts payable and accrued liabilities	75,049,000	89,994,000	62,319,000
Change in other current liabilities			(472,000)
Change in other liabilities	(139,000)	887,000
Net cash provided by operating activities	933,965,000	775,798,000	440,820,000
Cash flows from investing activities:
Payments to Explore and Develop Oil and Gas Properties	(1,354,281,000)	(893,524,000)	(1,051,365,000)
Acquisition of oil and gas properties	(46,247,000)	(1,560,072,000)
Proceeds from sale of properties	324,852,000
Costs related to sale of properties	(2,337,000)
Derivative settlements	6,774,000	(8,133,000)	6,545,000
Advances from joint interest partners	(6,213,000)	(8,347,000)	12,112,000
Net cash used in investing activities	(1,077,452,000)	(2,470,076,000)	(1,032,708,000)
Cash flows from financing activities:
Proceeds from revolving credit facility	620,000,000	600,000,000
Principal payments on revolving credit facility	(455,570,000)	(264,430,000)
Debt issuance costs	(99,000)	(6,548,000)	(705,000)
Investment in / capital contributions from subsidiaries	(33,433,000)	1,343,230,000	644,853,000
Net cash provided by financing activities	130,898,000	1,672,252,000	644,148,000
Decrease in cash and cash equivalents	(12,589,000)	(22,026,000)	52,260,000
Beginning of period	57,624,000	79,650,000	27,390,000
End of period	45,035,000	57,624,000	79,650,000
Intercompany Eliminations [Member]
Cash flows from operating activities:
Net income	(613,601,000)	(299,459,000)	(202,924,000)
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in earnings of subsidiaries	613,601,000	299,459,000	202,924,000
Working capital and other changes:
Change in accounts receivable	82,370,000	3,253,000	4,225,000
Change in accounts payable and accrued liabilities	(82,370,000)	(3,253,000)	(4,225,000)
Net cash provided by operating activities			$ 0

The total of the component of interest expense representing the noncash expenses charged against earnings in the period to amortize the long-term deferred financing costs and other income (expense) included in net income that results in no cash inflows or outflows in the period. Includes noncash adjustments to reconcile net income (loss) to cash provided by (used in) operating activities that are not separately disclosed.

No definition available.