Okla. Stat. tit. 16, § 23

In all cases where an action is brought against a grantee to recover real estate conveyed to him by warranty deed he must notify the grantor, or person bound by the warranty, that such suit has been brought, at least twenty (20) days before the day of trial, which notice shall be in writing and shall request such grantor or other person to defend against such action; and in case of failure to give such notice there shall be no further liability upon such warranty, except when it is clearly shown that it was impossible to make service of such notice.