Okla. Stat. tit. 16, § 16-61: Definitions

For the purposes of this act: (a) An interest in real estate

shall include, but not be limited to mortgage liens, interests of

purchasers under contract of sale, leases, easements, oil and gas

leases, and mineral and royalty interests. (b) A purchaser for

value shall include one who has actual or constructive notice of the

invalidity of the conveyance, decree or judgment under which his

grantor claims immediately or remotely.