B737-900
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown
Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Aircraft; Human Factors
Approximately 3.5 hours into [the] flight the First Officer (FO) reached for his O2 mask and realized we had no [flight crew] O2; the gauge read zero. I asked the A flight attendant to bring an O2 bottle to the flight deck. The FO and I started to search our [tablets] for company guidance on this type of event. While the FO searched I made a sat call to our dispatcher for guidance on this issue. Dispatcher sent me a flight plan at FL250. With this information elected to descend to FL250 for remainder of the flight. The masks were stowed from [departure] to the time we took our break. Both FO and I recall testing our masks and determined that we had about 1;500 PSI O2 prior to departure. Remainder of flight uneventful; further evidence we never know where treats may present themselves and to remain vigilant. The event was handled well; with good guidance from [flight operations]; Dispatcher; and crew to minimize the other threats an event like this can cause. Not sure; one of the crew masks may not have been stowed properly.
A B737-900 Captain reported noticing about halfway through a transcontinental flight that the flight crew oxygen showed zero pressure. The flight descended to FL250 for the balance of the trip.