Okla. Stat. tit. 68, § 68-1902: Exemption from other taxes

All mortgages of real property situated within this state which

are taxed by this article, and the debts and obligations which they

secure, together with the paper writings evincing the same, shall be

exempt from ad valorem and all other taxation by the state,

counties, towns, cities, school districts and other local

subdivisions of the state, except this article shall not affect in

any manner the collection of any income tax payable in whole or in

part from the interest received from such mortgage indebtedness. The

exemption conferred by this exemption shall not be construed to

impair or in any manner affect the purchaser of real estate which

may be sold for nonpayment of taxes levied by any local authority.