Mo. Rev. Stat. § 491.380: Competency of witnesses, how determined — oath of witnesses.

1. If a witness, on being produced, shall be objected to as incompetent, such objection shall be tried and determined by the associate circuit judge.

2. Every person offered as a witness, before any testimony shall be given by him, shall be duly sworn, or affirmed, that the evidence he shall give relating to the matter in issue between ______, plaintiff, and ______, defendant, shall be the truth, the whole truth, and nothing but the truth.