As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHX - AIR TURNBACK - FLT 0332 - EN ROUTE FROM PHX TO SJD WHILE CLIMBING TO CRUISE ALTITUDE FLIGHT CREW NOTICED CREW OXYGEN GAUGE READING BELOW 200 PSI, OXYGEN MASKS WERE CHECKED AND VERIFIED WITH NO PRESSURE, AIRCRAFT TURNED BACK TO PHX AND LANDED UNEVENTFULLY. MAINTENANCE VERIFIED OXYGEN GAUGE ON FLIGHT DECK AND AT CYLINDER BOTH INDICATED 0 PSI, REMOVED AND REPLACED OXYGEN CYLINDER IAW T/C S8M35001.