Mo. Rev. Stat. § 540.110
The foreperson of every grand jury, from the time of his appointment to his discharge, shall be authorized to administer any oath, declaration or affirmation, in the manner prescribed by law, to any witness who shall appear before such grand jury, for the purpose of giving evidence in any matter cognizable by them. In addition to the usual oath, the foreperson, before such witness shall testify, shall administer to him or her the following oath:
Do you further solemnly swear, or affirm, that you will not after your examination here, directly or indirectly, divulge or make known to any person or persons the fact that this grand jury has or has had under consideration the matters concerning which you shall be examined, or any other fact or thing which may come to your knowledge while before this body, or concerning which you shall here testify, unless lawfully required to testify in relation thereto?
§ 540.110, RSMo