Okla. Stat. tit. 12, § 12-1148.5: Service of summons

The summons may be served as in other cases except that such

service shall be at least three (3) days before the day of trial,

and the return day shall not be later than the day of trial, and it

may also be served by leaving a copy thereof with some person over

fifteen (15) years of age, residing on the premises, at least three

(3) days before the day of trial; or, if service cannot be made by

the exercise of reasonable diligence on the tenant or on any person

over the age of fifteen (15) years residing on the premises, the

same may be served by certified mail with return receipt postmarked

at least three (3) days before the date of trial.