Date: 201505
Local time of day: 1801-2400
Location: ZZZZ.Airport, FO
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZZ
Aircraft component: Oxygen System/Crew
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted

Narrative: Upon arriving at the aircraft and performing our preflight duties; both the first officer and I noticed the crew oxygen pressure showed 510 PSI at 21 degrees Celsius. I remember this because we both looked up the limitations in the FM because this was lower than usual. That pressure is within limits for operation with two pilots and both masks tested normally. We departed. We had been in cruise approximately two and a half hours when I noticed the crew oxygen pressure gauge had dropped to zero. I attempted to test my mask; thinking it was a gauge malfunction but the mask harness would not even expand. The first officer and I immediately came to the conclusion that we needed portable oxygen bottles in the flight deck while we worked through this problem. The flight attendants brought two of the bottles to the cockpit. I attempted to have dispatch contact me but he was unable to make a connection via [the data link provider]. We started working the problem in consultation with dispatch and maintenance. We were exchanging ideas on the best course of action via ACARS text but this was made more difficult with the ACARS dropping in and out of 'ACARS NO COMM' I attempted a phone patch through commercial radio using both VHF and HF and while the patch was made; neither the dispatcher nor I could hear or understand each other. I reverted back ACARS text messages. There was some thought about descending to 10;000 MSL and trying to make ZZZ but with the weather south of ZZZ; having to fly over the Gulf of Mexico and having the ability to cope with a smoke/fumes event severely limited; it was soon evident that the best course of action was a diversion. Both the dispatcher and maintenance agreed that a diversion was the best course of action given it is a large station; has maintenance and personnel available at that time of the morning. The first officer and I discussed the impending over weight landing and decided that it was best to land over weight than burn the fuel off over Mexican airspace; in the middle of the night with no oxygen for the flight deck. We landed uneventfully in ZZZZ. We did not declare an emergency or ask for expeditious handling. While in ZZZZ we complied with the provisions of the large tarmac delay program and kept dispatch informed of that fact via ACARS. I also made regular announcements to keep the passengers informed. It should be noted that the lead flight attendant did a superb job during the ground delay. He kept the passengers as comfortable as could be expected and was a great asset. After having the crew oxygen bottle replaced we departed for [intended destination] and arrived without further incident.


Second narrative: During preflight Pilot O2 bottle pressure indication 510 psi at oat 21 degrees C (in limits for 2 pilots). O2 mask checks were good.Approximately two and one half hours into cruise flight; Pilot O2 bottle pressure indication 0 psi. O2 mask checks confirmed 0 pressure. Portable Oxygen bottles brought to cockpit. Conferenced with dispatch and maintenance to determine best course of action. Lower altitude en route; fuel calculations and line of severe weather in vicinity ZZZ were some of the factors considered. Safest solution agreed upon by all parties was to divert into ZZZZ. Due to time to burn off fuel required; overweight landing was executed. Normal safe uneventful arrival and landing ZZZZ. No emergency declared. ZZZZ ops and maintenance responded in prompt; professional and timely manner. Successful maintenance performed on pilot O2 system and overweight landing check was good. Proper passenger care conducted by crew and ZZZZ ops. Flight continued on to ZZZ uneventful safe conclusion.


Synopsis: During cruise; B-737 crew noticed that the crew oxygen system was indicating zero. Crew obtained two portable oxygen bottles from the cabin and after conferring with Dispatch diverted to a closer airport. Cause of the loss of crew oxygen not reported.