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.Okla. Stat. tit. 16, § 61