As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON CLIMBOUT NOTED OXYGEN HAD DEPLETED FROM 1750 LBS TO 1000 LBS VERIFIED ALL REGULATORS / VALVES IN NORMAL FLIGHT POSITIONS ATTEMPTED TO TEST SYSTEM AND AFTER SEVERAL SHORT BREATHS OXYGEN SYSTEM DROP TO 0 PSI AND NO FLOW. UPON INSPECTION FOUND REGULATOR NUT LOOSE, SERVICED IAW B727 MM 12-15-11, OP'S CHECK GOOD AND NO OTHER DEFECTS NOTED.