Okla. Stat. tit. 12, § 63

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.