Date: 201807
Local time of day: 1201-1800
Location: ZZZZ.ARTCC, FO
Flight conditions: VMC
Light: Daylight
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Aircraft component: Oxygen System/Crew
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Troubleshooting; Time Pressure; Distraction; Confusion
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: We arrived to the [aircraft] that afternoon and MX (maintenance) were in the process of fixing the O2 mask receptacle on the FO (First Officer) side (the black bar that holds mask was loose). A swap with the observer O2 was the only solution. Once MX was done we were able to accomplish checks. Just prior to closing doors; they opened the valve and asked the flight deck to verify operation. Both CA (Captain) and I tested the O2 mask as per procedure; for proper flow and mike. We had the status page displayed and we observed a normal decrease of O2. I personally held it open for flow for 10 sec; only 10 psi decreased. Both CA and I were satisfied with the operation and O2 available.At FL360; right before the FA (Flight Attendants) were to start the service; they called the flight deck to give the CA the hot meal. I asked to do a quick bathroom break. It was approximately 1 hour in flight. It was when I was opening the door the CA; on pulling the O2 mask; noticed it did not inflate (the look on our faces had to be a good one). I looked at him puzzled; and I remember mentioning 'I know we had proper pressure; did they close the valve after we tested?'. CA told me 'go use the bathroom and I will figure this out'.I returned the flight deck and I saw on the status page that we had only 60PSI of O2! CA told me that had requested 'CALL ME to dispatch' as I took my seat I mentioned that we needed to get down to 10;000 feet ASAP (As Soon As Possible). CA wanted a QRH; I quickly searched Oxygen on the QRH iPad and found nothing related to loss of O2 from the cylinder. I advised the CA there was no procedure; but that we needed to get down quickly; we were not longer in [a] safe environment. CA was on patch to Dispatch and [Maintenance]; and finally said; yes ok request descend to 10;000 ft. CA was flying pilot; I was pilot monitoring. Once the CA was on the conversation with [Maintenance]; I had assumed I was flying and doing ATC; but CA never transferred airplane to me. I asked [ATC] for 10;000 feet and explained that we had lost the O2 supply for the flight deck. I set 10;000 feet and pointed to CA for verification. The CA started the descent to 10;000 feet using FLCH (Flight Level Change) and a slow speed. We were only descending at 1200 fpm. I mentioned to the CA we needed to divert; as [the nearest suitable airport] was only 1 hour away and best options per MX. CA was still in conversation to Dispatch and [Maintenance] in regards the lack of guidance and what to do. I pressed for a decision to divert and finally got the 'go-ahead' I requested new destination [with] ATC. Set the FMC to divert and set the new route on the FMC. CA was still on the radio with Dispatch. Dispatch coordinated the diversion with ZZZ and they turned off. I filled [in] the CA with all I had done; and then [ATC] asked us to squawk 7700. We explained to them the emergency would be over once we got to 10;000 feet and we would be able to continue to ZZZ. We were instructed to return to squawk at 10;000 ft. We continued with no further issues to ZZZ; landed 100 lbs below maximum landing weight. Taxied to gate and MX was waiting. The O2 mask in the FO position was cracked and the O2 leaked out inflight. A repair was made; new bottle was installed and we left a couple hours later to continue to [destination].




Synopsis: B757 First Officer reported a loss of crew oxygen system pressure in cruise resulted in a diversion to a suitable airport for maintenance.