As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PASSING THROUGH FL190 ON DECENT THE CAPTAIN OXYGEN MASK CAME ON IN EMERGENCY MODE. THE FLOW COULD NOT BE ARRESTED AND THE OXYGEN SYSTEM WAS DEPLETED. THE OXYGEN MASK AND OXYGEN BOTTLES WERE REPLACED. BOTTLES REPLACED DUE TO THE SYSTEM GOING TO 0 PSI PER AMM. WE HAD ANOTHER OXYGEN MASK ALTHOUGH DIFFERENT PN WITH THE SAME FAILURE. BOTH MASKS WERE OVERHAULED BY THE SAME VENDOR.