Mo. Rev. Stat. § 53.030: Oath.

Every assessor shall take an oath or affirmation to support the Constitution of the United States and of this state, and to demean himself faithfully in office and to assess all of the real and tangible personal property in the county in which he assesses at what he believes to be the actual cash value. He shall endorse this oath on his certificate of election or appointment before entering upon the duties of his office.