As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT ALTITUDE FL 35000 FT A HISSING NOISE WAS HEARD IN F/O''S 02 MASK RESULTING IN COCKPIT O2 BOTTLE GOING TO 0 PSI, QUANTITY WAS FINE IN CLIMBOUT AND INITIAL LEVEL OFF. FOUND DILUTER DEMAND REGULATOR TO PARTIALLY STAY IN TEST MODE THUS DEPLEATING O2 BOTTLE. REMOVED AND REPLACED BOTTLE WITH SERVICEABLE UNIT AND REMOVED AND REPLACED DILUTER DEMAND REGULATOR.