Okla. Stat. tit. 68, § 68-1901: Real estate mortgage defined

The words or term "real estate mortgage" as used in this article

shall be understood to include every species of conveyance intended

to secure the payment of money by lien upon real estate. Any

contract for the sale of real estate in which title is retained in

the vendor for the purpose of enforcing payment of the balance due

shall be deemed a mortgage upon real property. If an indebtedness

is secured by both real and personal property, said mortgage shall

be deemed to be a mortgage on real property for the purpose of this

article. Any contract or agreement by which the indebtedness

secured by any mortgage is increased or added to shall be deemed a

mortgage of real property for the purposes of this article and shall

be taxable as such upon the amount of such increase or addition.