Okla. Stat. tit. 58, § 58-41: Proceedings on contest

If anyone appears to contest the will, he must file written

grounds of opposition to the probate thereof, and serve a copy on

the petitioner and other residents of the county interested in the

estate, any one or more of whom may demur thereto upon any of the

grounds of demurrer allowed by law in civil actions. If the

demurrer be sustained, the court must allow the contestant a

reasonable time, not exceeding ten (10) days, within which to amend

his written opposition. If the demurrer is overruled, the

petitioner and others interested may jointly or separately answer

the contestant's grounds, traversing or otherwise obviating or

avoiding the objections. Any issues of fact thus raised, involving:

1. The competency of the decedent to make a last will and

testament.

2. The freedom of the decedent at the time of the execution of

the will from duress, menace, fraud, or undue influence.

3. The due execution and attestation of the will by the

decedent or subscribing witnesses; or,

4. Any other questions substantially affecting the validity of

the will must be tried and determined by the court.

On the trial the contestant is plaintiff, and the petitioner is

defendant.