No driver who has engaged in conduct prohibited by subpart B of this part shall perform safety-sensitive functions, including driving a commercial motor vehicle, unless the driver has met the requirements of part 40, subpart O, of this title. No employer shall permit a driver who has engaged in conduct prohibited by subpart B of this part to perform safety-sensitive functions, including driving a commercial motor vehicle, unless the driver has met the requirements of part 40, subpart O, of this title.