is this for the NG only conversion or would this also apply to the bi-fuel conversion?
Yes it is also for the Bi-fuel
For tax years beginning before January 1, 2015, a one-time income tax credit is available for 50% of the incremental cost of purchasing a new original equipment manufacturer AFV or converting a vehicle to operate on an alternative fuel. The state also provides a tax credit for 10% of the total vehicle cost, up to $1,500, if the incremental cost of a new AFV cannot be determined or when an AFV is resold, as long as a tax credit has not been previously taken on the vehicle. Equipment used for conversions must be new and must not have been previously used to modify or retrofit any vehicle. The alternative fuels eligible for the credit are compressed natural gas, liquefied natural gas, hydrogen, and liquefied petroleum gas (propane). Tax credits may be carried forward for up to five years. (Reference Oklahoma Statutes 68-2357.22)
CNG Conversion loans through Allegiance Credit Union
http://www.allegiancecu.org/loans/auto-loans/cng-conversion-loans.html