Mo. Rev. Stat. § 242.180

Each supervisor before entering upon his official duties shall take and subscribe to an oath before some officer authorized by law to administer oaths, that he will honestly, faithfully, and impartially perform the duties devolving upon him in office as supervisor of the drainage district in which he was elected, and that he will not neglect any of the duties imposed upon him by sections 242.010 to 242.690.