Ohio Rev. Code § 4549.14: Incompetency of officer as witness.

Any officer arresting, or participating or assisting in the arrest of, a person charged with violating the motor vehicle or traffic laws of this state, provided the offense is punishable as a misdemeanor, such officer being on duty exclusively or for the main purpose of enforcing such laws, is incompetent to testify as a witness in any prosecution against such arrested person if such officer at the time of the arrest was using a motor vehicle not marked in accordance with section 4549.13 of the Revised Code.