Mont. Code Ann. § 20-1-202

Any person elected or appointed to any public office authorized by this title shall take the oath of office before qualifying for and assuming the office. If an officer has a written appointment or commission, the officer's oath must be endorsed on the appointment or commission, otherwise the oath may be taken orally, and, in either case, it may, without charge or fee, be sworn to before an officer authorized to administer oaths for the public office.