XML 87 R90.htm IDEA: XBRL DOCUMENT v2.4.0.6

Supplemental Oil and Gas Reserve Information - Additional Information (Detail)	12 Months Ended				1 Months Ended		12 Months Ended
	Dec. 31, 2012 MBbls MBoe	Dec. 31, 2011 MBbls MBoe	Dec. 31, 2010 Property MBbls MBoe	Dec. 31, 2009	Nov. 30, 2010 Roosevelt County, Montana [Member] MBbls	Dec. 31, 2010 Richland County, Montana [Member] MBbls	Dec. 31, 2012 MBoe [Member] MBbls	Dec. 31, 2011 MBoe [Member] MBbls	Dec. 31, 2010 MBoe [Member] MBbls	Dec. 31, 2012 MBoe [Member] Burke and Mountrail counties [Member] MBbls
Reserve Quantities [Line Items]
Purchases of estimated net proved reserves in place					715	764	966	111	1,479	966
Extensions, discoveries and other additions	73,871	43,168	24,473				73,871	43,168	24,473
Estimated extensions and discoveries of reserves associated with new wells	16,548	12,696	8,122
Additional proved undeveloped reserves	57,322	30,472	16,351
Percentage of reserves produced from wells in certain formations	100.00%	100.00%	99.00%
Percentage of proved undeveloped reserves	100.00%	100.00%	100.00%
Number of traded interests in non-operated properties			3
Remaining reserves								(142)	(123)
Net positive (negative) revisions of previous estimates							(1,983)	(288)	2,552
Percentage of Net positive (negative) revisions	(2.50%)	(0.72%)
Percentage of revisions due to increase in oil prices			29.00%
Unweighted arithmetic average first-day-of-the-month prices for oil, price per Bbl	94.68	96.23	79.40	61.04
Percentage of revisions due to higher working interests in proved wells			29.00%
Remaining reserves			42.00%
Discount rate of future cash flows	10.00%
Unweighted arithmetic average first-day-of-the-month prices for natural gas, price per MMBtu	2.75	4.12	4.38

X

- Definition

Additional proved undeveloped reserves

+ References

No definition available.

+ Details

Name:	oas_AdditionalProvedUndevelopedReserves
Namespace Prefix:	oas_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Discount rate of future cash flows

+ References

No definition available.

+ Details

Name:	oas_DiscountRateOfFutureCashFlows
Namespace Prefix:	oas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Estimated extensions and discoveries of reserves associated with new wells

+ References

No definition available.

+ Details

Name:	oas_EstimatedExtensionsAndDiscoveriesOfReservesAssociatedWithNewWells
Namespace Prefix:	oas_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of traded interests in non-operated properties

+ References

No definition available.

+ Details

Name:	oas_NumberOfTradedInterestsInNonOperatedProperties
Namespace Prefix:	oas_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of proved undeveloped reserves.

+ References

No definition available.

+ Details

Name:	oas_PercentageOfProvedUndevelopedReserves
Namespace Prefix:	oas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of reserves produced from wells in certain formations.

+ References

No definition available.

+ Details

Name:	oas_PercentageOfReservesProducedFromWells
Namespace Prefix:	oas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of revisions due to higher working interests in proved wells

+ References

No definition available.

+ Details

Name:	oas_PercentageOfRevisionsDueToHigherWorkingInterestsInProvedWells
Namespace Prefix:	oas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of revisions due to increase in oil prices

+ References

No definition available.

+ Details

Name:	oas_PercentageOfRevisionsDueToIncreaseInOilPrices
Namespace Prefix:	oas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of revisions to estimates due to other changes

+ References

No definition available.

+ Details

Name:	oas_PercentageOfRevisionsToEstimatesDueToOtherChanges
Namespace Prefix:	oas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Revisions as a percentage of beginning of the years reserves balance.

+ References

No definition available.

+ Details

Name:	oas_RevisionsPercentage
Namespace Prefix:	oas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Unweighted arithmetic average first-day-of-the-month prices

+ References

No definition available.

+ Details

Name:	oas_UnweightedArithmeticAverageFirstDayOfMonthPrices
Namespace Prefix:	oas_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Unweighted arithmetic average first-day-of-the-month prices for natural gas

+ References

No definition available.

+ Details

Name:	oas_UnweightedArithmeticAverageFirstDayOfMonthPricesForNaturalGas
Namespace Prefix:	oas_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 5

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59F

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Purchase of minerals in place.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 5

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59F

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Revisions represent changes in previous estimates of proved reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 5

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59F

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Sales of minerals in place.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 5

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59F

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_ReserveQuantitiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration