Okla. Stat. tit. 36 § 4805
When any insurance policy subject to the provisions of this article contains a provision that the insured must render a written sworn proof of loss within sixty (60) days from the date of fire or loss to the insurer, or the same is required by law to be so rendered, the insurer cannot assert the failure of insured to so render such proof of loss in any litigation or court proceeding, unless the insurer plead and prove that it has furnished the insured with two blank forms for the execution of proof of loss, that has printed thereon, in bold-faced type in a conspicuous place, the warning that a proof of loss must be rendered to the insurer within sixty (60) days from the date of receipt of the blank forms for proof of loss by the insured, or by putting such warning in a like form in a letter of instruction for executing a proof of loss that will accompany the proof of loss blanks furnished the insured, and the insurer has further executed and furnished the insured its written extension of time, giving the insured sixty (60) days from the date such blanks were received by the insured. These requirements cannot be waived by any agreement between the parties or otherwise.
Okla. Stat. tit. 36, § 4805