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:
On the trial the contestant is plaintiff, and the petitioner is defendant.
Okla. Stat. tit. 58, § 41