As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF, CREW OBSERVED SMOKE COMING FROM OXYGEN REGULATOR IN COCKPIT. CREW DECLARED A RED EMERGENCY AND FLIGHT WAS RETURNED TO FSD. AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE FOUND OXYGEN LIGHT PLATE SHORTED OUT. REPLACED OBSERVER'S OXYGEN REGULATOR, OPERATIONAL CHECK NORMAL.