Okla. Stat. tit. 12, § 12-63: Real estate mortgage as bond

In every instance in this state where bond, indemnity or

guaranty is required, a first mortgage upon improved real estate

within this state shall be accepted: Provided, that the amount of

such bond, guaranty or indemnity shall not exceed fifty percent

(50%) of the reasonable valuation of such improved real estate,

exclusive of all buildings thereon; Provided, further, that where

the amount of such bond, guaranty or indemnity shall exceed fifty

percent (50%) of the reasonable valuation of such improved real

estate, exclusive of all buildings, then such first mortgage shall

be accepted to the extent of such fifty percent (50%) valuation.