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.
R.L. 1910, § 5344.