As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER DEPARTURE FROM SFB, CREW FOUND OXYGEN WAS DEPLETED. AIRCRAFT RETURNED TO SFB WITHOUT INCIDENT. THE FLIGHT ENGINEERS REGULATOR SWITCH WAS FOUND TO BE IN THE ON POSITION. RETURNED THE FLIGHT ENGINEERS REGULTOR SWITCH TO THE OFF POSITION AND SERVICED THE CREW OXYGEN. ALL OPERATIONAL AND LEAK CHECKS WERE GOOD.