B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 8846
Situational Awareness
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action
Aircraft; Human Factors; Procedure
After arriving to the aircraft; we began our preflight duties and flows. Upon checking my oxygen mask; I ran the oxygen via the test button for 5 seconds and made sure the microphone worked. However; I immediately saw the O2 PSI go from 1600 to 100 instantly and stay there. When I released the test button the gauge went back to 1600 PSI. I suspected the valve on the bottle itself was in the off position. I told the captain and he agreed. We made an Electronic Log Book write-up and backed it up with a call to maintenance. They came to the plane and the mechanic was shocked. I also believe the bottle might have been slightly open as the gauge did go back to 1600 PSI. However; this would not have been sufficient in a smoke/fire/depressurization emergency.The mechanic turned the bottle on. The test worked normally and we received a new [maintenance release]. The flight continued to [destination].I was concerned as the plane had only been in ZZZ for 20 minutes when we arrived. The [maintenance release] was from ZZZ1 and that indicated to me that the plane flew in that condition from ZZZ1 to ZZZ. I am extremely concerned and want to make sure that we are all safe; as well as our passengers. This is the type of maintenance condition that could have dire consequences if not caught before a flight; and things don't go as planned.
B737 First Officer reported that the crew oxygen system failed to test properly. Maintenance found the valve on the oxygen bottle was not fully open.