As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
COCKPIT O2 BOTTLE HAD GOOD PRESSURE DURING PREFLIGHT. ON TAXI OUT CREW HEARD HISSING NOISE AND FOUND O2 BOTTLE PRESSURE DEPLETING. PRESSURE GAGE READ ZERO BY THE TIME RETURNED TO GATE. INSTALLED NEW O2 BOTTLE IAW AMM. STILL LEAKING. REPLACED OBSERVERS O2 MASK IAW AMM. LEAK AND OPERATION CHECKED GOOD.