Date: 202012
Local time of day: 1201-1800
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Cruise
Airspace: Class B ZZZ
Aircraft component: Oxygen System/Crew
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Procedure; Aircraft
Primary problem: Aircraft

Narrative: During a reposition flight from ZZZ to ZZZ1 we had experienced a Master Caution indicating a wing body overheat. We complied with the appropriate procedures per the QRH. About 20 minutes later; we received another alert; this time for CABIN ALT high. I began to execute the memory items procedure for such an event but that procedure was interrupted when we realized there was no ABO (Aviator's Breathing Oxygen) available for use. Captain asked ATC to descend to lower altitude and I had asked the mechanic to bring me an oxygen bottle from the back; which I used. When we initiated the descent we had determined that cabin altitude was controllable.At FL360 we had experienced a Master Caution indicating a wing body overheat. We complied with the appropriate procedures per the QRH. About 20 minutes later; we received another alert; this time for CABIN ALT high. I began to execute the memory items procedure for such an event but that procedure was interrupted when we realized there was no ABO available for use. During pre-flight; after performing the walk-around; I had arrived to the flight deck having incorrectly assumed that all pre-flight inspection procedures were completed. When we came to the pre-flight portion of the checklist; I pushed the oxygen test button on the FO (First Officer) side mask; and remember hearing a sound but I do not remember looking at the Oxygen Quantity Indicator like I always do. It is possible that we took off with an appropriate oxygen supply; and that it somehow changed during the flight. It is also possible that we simply took off without an appropriate oxygen supply.Captain asked ATC to descend to lower altitude and I had asked the mechanic to bring me an oxygen bottle from the back; which I used. When we initiated the descent we had determined that cabin altitude was controllable and no further incident had occurred enroute to ZZZ1 thereafter.We understand that the ABO may have been serviced for this particular aircraft. I recommend that every time such a procedure has been performed; that it be noted in the aircraft logbook so that the flight crew are aware. While I certainly would never not do a thorough preliminary pre-flight inspection myself again; if this suggestion is appropriate; I recommend a change to the FOM (Flight Operating Manual) that the preliminary pre-flight procedure be performed by the Captain and the FO.




Synopsis: Air carrier flight crew reported after receiving a Wing/Body Overheat warning and securing the Bleed Air Source; it was discovered that there was no Crew Oxygen left to use because of a loss of cabin pressure.