Okla. Stat. tit. 12, § 1458

If no answer be made, a peremptory mandamus must be allowed against the defendant; if answer be made, containing new matter, the same shall not, in any respect, conclude the plaintiff, who may, on the trial or other proceeding, avail himself of any valid objections to its sufficiency, or may countervail it by proof, either in direct denial or by way of avoidance.