Sec. 121.333 - Supplemental oxygen for emergency descent and for first aid; turbine engine powered airplanes with pressurized cabins.SNIP... (3) Notwithstanding paragraph (c)(2) of this section, if for any reason at any time it is necessary for one pilot to leave his station at the controls of the airplane when operating at flight altitudes above flight level 250, the remaining pilot at the controls shall put on and use his oxygen mask until the other pilot has returned to his duty station.
Do not apply to foreign carries that are operating outside of the United States.