Okla. Stat. tit. 82, § 1613
Any use that exists prior to the effective date of this act which does not meet the minimum standards set forth herein may continue. However, unless brought into compliance with the minimum standards set forth in regulations adopted pursuant to this act such uses may not be substantially altered, enlarged or added to.
Laws 1980, c. 179, § 13, emerg. eff. May 13, 1980.