Okla. Admin. Code § 310:2-21-6
Any party served with a petition, an application for an administrative fine, an administrative order or other instrument providing notice of a claim or defense to a claim initiating an individual proceeding before the Department shall file a written response or answer within twenty (20) days of receipt of the petition, application, order or other instrument initiating an individual proceeding. The original of the response must be filed with the Hearing Clerk of the Office of Administrative Hearings and a copy must be delivered or mailed to all other parties by 5:00 p.m. on the 20th day. Delivery to other parties must be made in person, by process server, or may be sent by certified mail, return receipt requested, restricted delivery. Every defense, in law or fact, to a claim for relief in any petition, application or administrative order initiating an administrative proceeding shall be asserted in the responsive pleading.
Okla. Admin. Code § 310:2-21-6