B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Type 4066
Aircraft Equipment Problem Less Severe
Flight Crew Diverted; General Maintenance Action
Aircraft
Preflight of mask and oxygen indicated normal system. I addressed the supply at 750 PSI to First Officer; but said it was good to go. About 2.5 hours into flight I noticed oxygen level was 200 PSI. At that point I contacted Dispatch and got [Maintenance Control] phone patch. The oxygen level continued to drop even though we were not using masks. As oxygen was approaching zero; [advised ATC] and requested divert to [an alternate airport]. I could still hear Dispatch on [Commercial Radio] frequency saying not to divert to [a nearby airport] because of thunderstorms hitting field. We also received an ACARS message saying not to use [the nearby airport]. At lower altitude I could no longer speak with Dispatch on [Commercial Radio] Frequency. Divert to [the alternate airport] accomplished without incident. Contract maintenance checked cockpit oxygen and verified zero remaining. We elected to descend; divert and [advise ATC] because of no oxygen for use in decompression or smoke/fire in cockpit.
B737NG Captain reported problems with crew oxygen quantity.