Okla. Stat. tit. 73, § 73-83.6: Existing uses, buildings and structures

A legally existing use, building or structure, existing at the

time of the adoption and recording of any regulations authorized

hereunder, but not in conformity therewith, may be continued but

shall not be extended or structurally altered unless the same be

changed to conform to such regulations or changed to a higher or

more restrictive use.