Okla. Stat. tit. 68, § 500.62

The tax imposed by Section 4 of this act shall be applicable to all nonexempt inventory held by any person outside of the bulk transfer system in this state in quantities which, in the aggregate with respect to such person, exceed one thousand (1,000) gallons, to the extent the inventory has not previously been subject to the tax imposed by this state under the predecessor motor fuel tax statute. However, no tax shall be payable with respect to motor fuel which is dyed diesel fuel or held by an exempt user. The inventory tax imposed on inventory held outside of the bulk transfer system on the effective date of this act reportable under this section shall be payable in two equal annual installments beginning twelve (12) months after the effective date of this act.