As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHX - APPROXIMATELY 1 HR INTO FLIGHT, STATUS PAGE INDICATED O2 PRESS OF 110 NR. THE CREW DECLARED AN EMERGENCY AND RETURNED TO PHX. THE PAX DID NOT BRACE. THE LANDING WAS UNEVENTFUL. MAINTENANCE REMOVED AND REPLACED THE O2 BOTTLE PER TC NR R2A35601. NO LEAKS NOTED. MAINTENANCE REMOVED REPLACED THE CAPTAIN'S AND FIRST OFFICER'S O2 MASK PER TC NR R1B35601. OBSERVED SYSTEM PRESSURE FOR 30 MINUTES, NO DEFECTS NOTED. MASK, O2 - FIRST OFFICERS, P/N 174660N5 - S/N 6508 - TSR 2113.