Okla. Stat. tit. 69, § 4025

A. The Department of Transportation shall issue rules and regulations in accordance with the Administrative Procedures Act, Section 301 et seq. of Title 75 of the Oklahoma Statutes, to implement this program. B. Pursuant to subsection A of this section, the Oklahoma Department of Transportation shall not prevent sign placement by contract or by rule in any county with a population of less than five hundred thousand (500,000) people according to the most recent Federal Decennial Census; provided however, nothing in this section shall prohibit the Department from allowing such sign placement in any county with a population of more than five hundred thousand (500,000) people.