Date: 201604
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Aircraft: B737-900
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Oxygen System/Crew
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On Takeoff roll; the First Officer (FO) and I noticed an 'air whooshing' sound around Vr. We both instinctively pushed forward on our window handles; and he said 'that's an odd sound'. With no warning lights; we continued into the air and cleaned up the configuration of the aircraft. The After Takeoff Checklist showed no abnormalities and the pressurization was working normally. We continued on to FL 350.About 10 minutes prior to Top of Descent (TOD); at FL 350; FO and I decided to coordinate a bathroom break with the flight attendants. As FO was getting ready to leave the flight deck; I pulled out my O2 mask and noticed the secure straps would not inflate and there was no flow sound when I pulled out my mask. It was then that I realized what the 'air whooshing' noise was on takeoff. We checked the flight deck O2 and the gauge read zero. We decided to start a slow descent to FL 250 while we looked up any guidance from the QRH and MEL. We also sent dispatch a message that 'Flight Deck O2 quantity indicated Zero and we were descending to FL 250. Do you know of anything else we should consider?'As we were leveling at FL250; ATC told us to continue to FL 240. Dispatch sent a message telling us there was no guidance or checklist for this situation. Within a couple of minutes; we got cleared to descend and continued our descent to below 10;000 feet to 5;000 feet.During the descent; we notified the flight attendants of our lack of O2 in the flight deck and asked if we could place a Portable O2 bottle with two mask in the flight deck to be prepared; in the case we had a rapid depressurization or smoke/fumes.  I contacted [personnel in flight operations] and advised him of the incident and asked if he would let me know what maintenance found with the O2. He contacted me with the info that the O2 flight deck bottle had been removed and replaced and the aircraft continued to [the next destination] with the next crew. Some considerations for this incident:There is NO checklist for flight deck O2 going to zero in flight (it has always been checked on the ground and if it were zero at that time; we would not go anywhere)If this situation had been noticed earlier in flight; according to the FARs; we would have had to descend right away. It was a clear VFR day and we had no alternate fuel. The lower altitude would have a much higher fuel burn; which would have necessitated a divert. This situation could have worse fuel consequences if on [oceanic route] or Trans-continental flight.After realizing we had no O2 in the flight deck; we asked the flight attendants to bring us a portable bottle with 2 mask in the event we had a rapid depressurization. With the new full face O2 mask; smoke goggles have been removed. We realized that if a smoke/fumes situation would have developed; the portable O2 bottle with 2 mask would not have been a good situation.


Second narrative: On takeoff; we both noticed a strange airflow noise at about 200 ft AGL. We both checked that our windows were closed and locked; and confirmed that the aircraft was pressurizing. Our primary focus shifted back to flying/monitoring the aircraft on the published departure. In a short time the noise diminished and went away. We continued our climb to cruise at FL360. Shortly before the top of descent; we arranged for a relief break. When I tried to pull my oxygen mask out; it wouldn't inflate. Looking up; I saw that crew oxygen was at 0 psi. We requested a descent to FL240 and began looking for guidance in our supplements. We also sent dispatch a message; advising them of the situation and asking them to advise us if we needed to descend further. We asked the flight attendant to bring the portable oxygen bottle to the cockpit; with two masks. We felt that this would provide us with some supplemental oxygen in case of a pressurization problem or smoke/fumes situation. We found that there is no guidance in either the aircraft Quick Reference Handbook or the company publications. Dispatch responded that they could not find any guidance either. During this time; we were cleared to descend via the arrival into our destination and descended as published below 10;000 ft MSL. I am submitting this report to record what happened and point out that there is no guidance readily available to crews in this situation. I feel that we did the best we could to comply with FAR supplemental oxygen requirements and to ensure the safety of our passengers in case of an emergency.


Synopsis: B737-900 flight crew reported that while in flight the Crew Oxygen had lost all its pressure.  He mentioned he and FO did notice an air wooshing sound on takeoff.