Mo. Rev. Stat. § 65.160: Oath — shall assume office, when.

Every person chosen or appointed to the office of township trustee and ex officio treasurer, member of the township board, or township clerk, before such person enters on the duties of his or her office and within ten days after such person shall be notified of his or her election or appointment, shall take and subscribe, before any officer authorized to administer oaths, such oath or affirmation as is prescribed by law.