B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 120; Flight Crew Total 16500; Flight Crew Type 5200
Situational Awareness; Communication Breakdown; Confusion; Training / Qualification
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Flight Crew Overcame Equipment Problem; General Maintenance Action
Procedure; Human Factors; Aircraft
During preflight safety check of crew oxygen system; crew suspected closed O2 valve. Wrote it in log book. Maintenance checked valve and confirmed open. Captain checked flow on mask and noted no change on O2 pressure gauge. Confirmed logbook write up.  In flight I was going to take a bathroom break so First Officer cleared his O2 mask and checked the pressure gauge at the same time and noted pressure went to zero. Seconds later the pressure returned to 950PSI. I suspected that the valve had either not been opened or subsequently closed. I wrote that in the logbook. Prior to that I requested an early descent into ZZZ from ATC. I canceled the bathroom break and had flight attendants bring two oxygen bottles into cockpit.  Landed uneventfully. Consulted with mechanic who said that he found the O2 bottle closed; but opened a quarter turn. Advised outbound crew. System Malfunction.
B737 Captain reports the First Officer (F/O) cleared his O2 mask and noted the O2 pressure dropped to zero in flight. Seconds later the pressure returned to 950 PSI. Arrival station Aircraft Maintenance Technician (AMT) found O2 bottle handle only one-quarter turn opened. Captain had made logbook write-up at previous station suspecting bottle was 'closed'; but was assured the O2 handle was 'open'.