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13. OPTIONS AND WARRANTS - Summary of Warrant Outstanding and Exercisable (Detail) (Warrant Share [Member], USD $)
 12 Months Ended
Dec. 31, 2014
Range Five [Member]
  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Exercise Price Minimum, warrant $ 1.21opxa_ShareBasedCompensationSharesAuthorizedUnderOtherThanOptionConversionPriceRangeLowerRangeLimit
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeFiveMember
Exercise Price Maximum, warrant $ 4.99opxa_ShareBasedCompensationSharesAuthorizedUnderOtherThanOptionConversionPriceRangeUpperRangeLimit
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeFiveMember
Weighted Average Remaining Contractual Life (years), warrant 2 years 7 days
Number of warrants Outstanding 2,409,033us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeFiveMember
Number of warrants Exercisable 2,409,033opxa_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableNumbers
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeFiveMember
Range Six [Member]
  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Exercise Price Minimum, warrant $ 5.00opxa_ShareBasedCompensationSharesAuthorizedUnderOtherThanOptionConversionPriceRangeLowerRangeLimit
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeSixMember
Exercise Price Maximum, warrant $ 10.44opxa_ShareBasedCompensationSharesAuthorizedUnderOtherThanOptionConversionPriceRangeUpperRangeLimit
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeSixMember
Weighted Average Remaining Contractual Life (years), warrant 2 months 8 days
Number of warrants Outstanding 637,768us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeSixMember
Number of warrants Exercisable 637,768opxa_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableNumbers
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeSixMember
Range Seven [Member]
  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Exercise Price Minimum, warrant $ 1.21opxa_ShareBasedCompensationSharesAuthorizedUnderOtherThanOptionConversionPriceRangeLowerRangeLimit
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeSevenMember
Exercise Price Maximum, warrant $ 10.44opxa_ShareBasedCompensationSharesAuthorizedUnderOtherThanOptionConversionPriceRangeUpperRangeLimit
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeSevenMember
Weighted Average Remaining Contractual Life (years), warrant 2 years 2 months 16 days
Number of warrants Outstanding 3,046,801us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeSevenMember
Number of warrants Exercisable 3,046,801opxa_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableNumbers
/ us-gaap_ClassOfWarrantOrRightAxis
= opxa_WarrantShareMember
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= opxa_RangeSevenMember