As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING THE ENROUTE CLIMB, PERFORMED THE 10,000 FOOT PORTION OF CLIMB CHECKLIST. WHEN TESTING THE O2 MASKS, DISCOVERED THAT NEITHER CREW O2 MASK HAD OXYGEN FLOW. DUE TO LOSS OF THE SUPPLEMENTAL OXYGEN SYS, ELECTED TO RETURN TO BASE. LANDED WITHOUT INCIDENT. UPON LANDING, THE O2 VALVE IN THE NOSE OF THE ACFT WAS EXAMINED AND FOUND TO BE SLIGHTLY OUT OF THE "ON" DETENT.