Mo. Rev. Stat. § 86.220

Each trustee shall, within ten days after his appointment or election, take an oath of office before the clerk of circuit court of said cities, that, so far as it devolves upon such trustee, the trustee will diligently and honestly administer the affairs of the said board, and will not knowingly violate or willingly permit to be violated any of the provisions of the law applicable to the retirement system. Such oath shall be subscribed to by the trustee making it and certified by the said clerk of circuit court and filed in the clerk's office.