Ohio Rev. Code § 2939.13: Oath of witnesses.

Before a witness is examined by the grand jury, an oath shall be administered to him by the foreman of the grand jury or by the judge of the court of common pleas or the clerk of the court of common pleas, truly to testify of such matters and things as may lawfully be inquired of before such jury. A certificate that the oath has been administered shall be indorsed on the subpoena of the witness or otherwise made by the foreman of the grand jury, judge, or clerk certifying the attendance of said witness to the clerk of the court.