Okla. Stat. tit. 68 § 1906
If the principal indebtedness secured or which by any contingency may be secured by a mortgage is not determinable from the terms of the mortgage, or if a mortgage is given to secure the performance by the mortgagor, or of any other person of a contract obligation other than the payment of a specific sum of money and the maximum amount secured or which by any contingency may be secured by the mortgage is not expressed therein, such mortgage shall be taxable upon the value of the property covered by the mortgage, which shall be determined by the county treasurer to whom such mortgage is presented for taxation, unless at the time of presenting such mortgage for taxation the owner thereof shall file with the county treasurer a sworn statement of the maximum amount secured by the mortgage. If such maximum amount is expressed in the mortgage or in a sworn statement filed as required by this section, such amount shall be the basis for assessing the tax levied by this article. The statement filed by the owner of a mortgage pursuant to this section shall thereafter at all times be binding upon and conclusive against such owner, the holders of any bonds or obligations secured by such mortgage and all persons claiming through the mortgagee any interest in the mortgage or the mortgaged premises. If the maximum amount secured or which by any contingency may be secured by the mortgage is not expressed in the mortgage or in a sworn statement so authorized by this section, the county treasurer at the time such mortgage is offered for taxation may require the mortgagor or mortgagee to furnish him with proofs as to such facts as he deems necessary for the purpose of computing the value of the property covered by the mortgage, and such proofs shall be preserved in his office. His determination as to the basis for computing the tax on such mortgage shall be subject to review on appeal to the district court under the same procedure as cases appealed from the county commissioners to the district court.
Okla. Stat. tit. 68, § 1906