Okla. Stat. tit. 12, § 12-1142: Actions to recover real property

In actions for the recovery of real property, it shall be

necessary for the plaintiff to set forth in detail the facts relied

upon to establish his claim, and to attach to his petition copies of

all deeds or other evidences of title, as in actions upon written

contracts; and he must establish the allegations of his petition,

whether answer be filed or not.