Okla. Stat. tit. 47 § 1-173.1
Any commercial motor vehicle designed to transport any liquid or gaseous materials within a tank or tanks having an individual rated capacity of more than one hundred nineteen (119) gallons and an aggregate rated capacity of one thousand (1,000) or more gallons that is either permanently or temporarily attached to the vehicle or the chassis. A commercial motor vehicle transporting an empty storage container tank, not designed for transportation, with a rated capacity of one thousand (1,000) or more gallons that is temporarily attached to a flatbed trailer is not considered a tank vehicle. Such vehicles include but are not limited to cargo tanks and portable tanks as defined by 49 C.F.R., Part 171 ; provided, a commercial motor vehicle transporting an empty storage container tank, not designed for transportation, with a rated capacity of one thousand (1,000) or more gallons that is temporarily attached to a flatbed trailer is not considered a tank vehicle.
Okla. Stat. tit. 47, § 1-173.1