Okla. Stat. tit. 58 § 43
If the will is contested, all the subscribing witnesses who are present in the county, and who are of sound mind, must be produced and examined; and the death, absence or insanity of any of them must be satisfactorily shown to the court. If none of the subscribing witnesses reside in the county, and are not present at the time appointed for proving the will, or although such witnesses reside in the county and are insane or incompetent, and such facts are first made to appear to the court, either in contested or noncontested will cases, the court may admit the testimony of other witnesses to prove the sanity of the testator and the execution of the will and, as evidence of the execution, it may admit proof of the handwriting of the testator and of the subscribing witnesses, or any of them. Provided that when the testimony of any nonresident witness or witnesses residing out of the county wherein any will is sought to be admitted to probate, may be desired, touching the execution of such will, either in contested or noncontested will cases, it shall be lawful for the party seeking to have such will admitted to probate, or resisting the same in the district court, to cause the deposition of such witness to be taken in like manner, as now is or hereafter may be provided in civil cases; and the court may, in its discretion, direct the original of such will to be attached to any commission issued in such case; and the deposition of any such witness taken, certified and returned, according to law, shall be of like force and effect as if his testimony had been heard in the court; provided, that before any such original will shall be suffered to be attached to any such commission, a photostatic or certified copy thereof shall be made and examined, and certified by the judge to be a true copy of the original, and until the return of such original, such copy shall be retained in the office of the judge, in lieu of such original will; and if such will be admitted to probate, the same may, in case of the loss or destruction of the original thereof, be recorded from such certified copy. Provided, further, that in all cases where wills have heretofore been proved in substantial compliance with the provisions hereof, such proof is hereby validated.
Okla. Stat. tit. 58, § 43