Okla. Stat. tit. 68 § 1909
When property is in more than one county, or when the real property covered by a mortgage is assessed in more than one county, it shall be the duty of the county treasurer of the county where said mortgage is offered for taxation to ascertain the assessed value of the property in each county and to apportion the amount upon which the tax shall be paid to the county treasurer in each of the said counties upon the basis of the relative assessments. Where the mortgage is a first lien upon the real property situate in another county, it shall be his duty to apportion the amount of the tax property to be credited to said county by ascertaining the valuation of each parcel as appears from the last preceding assessment roll of the county in which such parcel is located, after deducting therefrom the taxable amount of any prior lien. If, however, the whole or a part of the property covered by the mortgage in a county is not assessed in the last preceding assessment roll or rolls of said county in which it is located, or is assessed as a part of a larger tract in such a manner that the assessed value cannot be determined from the assessment rolls or roll, or improvements have been made upon the property so assessed, the county treasurer may determine the value of the property covered by the mortgage and for such purpose may require the mortgagor or mortgagee to furnish him with proofs as to such facts as he deems necessary for the purpose of computing such value, and the value so determined shall be deemed to be the assessed value for the purpose of such apportionment. When the real property covered by a mortgage is located partly within the state and partly without the state, it shall be the duty of the county treasurer to whom said mortgage is offered for taxation to determine what proportion shall be taxable under this article by determining the relative value of the mortgaged property within this state as compared to the total value of the entire mortgaged property, taking into consideration in so doing the amount of all prior encumbrances upon such property or any portion thereof. If a mortgage covering property located partly within the state and partly without the state is presented for taxation before such determination has been made, then there may be presented to the recording officers, with such mortgage, or at the time when the first advance is made on prior advance mortgage as provided in Section 1911 of this article, a statement in duplicate verified by the mortgagor or an officer or a duly authorized agent or attorney of the mortgagor, specifying the value of the property covered by the mortgage within the state and the property covered by the mortgage without the state, stated separately. Such statements shall be filed with the county treasurer. The tax payable under this article shall be computed upon such properties of the principal indebtedness secured by the mortgage or of the sum advanced thereon, as the case may be, as the value of the mortgaged property within the state shall bear to the total value of the entire mortgaged property, as set forth in such statement. In determining the separate values of the property covered by any such mortgage within and without the state for the purpose of ascertaining the proportion of the principal indebtedness secured by the mortgage which is taxable under this Article, the county treasurer shall consider only the value of the tangible property covered by each mortgage, taking into consideration in so doing the amount of all prior encumbrances thereon. For the purpose of determining such value the county treasurer may require the mortgagor or mortgagee to furnish him by affidavit or verified report such information or data as he deems needed for the purpose, or he may take the testimony of the mortgagor or any other person in relation thereto, and if any person whose testimony is desired can be found within the state, may require him by subpoena to attend before him at a specified time and place for the purpose of testifying in relation to the value of said property. He may also determine at the same time the proportion of the tax which shall be paid by the county treasurer who has received the same to the several county treasurers of the respective counties in the state in which parts of the mortgaged property are situated. When such county treasurer shall pay any portion of such tax to the county treasurer of any other county, he shall at the same time file in the office of the county clerk of such county a brief description of the mortgage on which such tax is paid sufficient to identify the same, together with a statement of the payment of such tax, and the amount thereof, and the county clerk of such other county shall note on the margin of the record of such mortgage the fact of such payment, attested by his signature.
Okla. Stat. tit. 68, § 1909