B737 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 146; Flight Crew Total 10052; Flight Crew Type 10052
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem
Aircraft
We got onto this aircraft as part of a normal pairing. The airplane had come in from ZZZ about 1 hour before we got to it. When preflighting the oxygen masks; the pressure dropped about 500 psi. I called maintenance and made an ELB (Electronic Log Book) entry. The first mechanic told me that was normal. I told him that it was not and would not take the airplane!A more knowledgeable mechanic came up and immediately knew that the valve wasn't open!He went into the cargo compartment and opened the valve. Oxygen masks tested normal.When the mechanic came to this cockpit; he told me the valve was half shut and the safety wire was broken.We looked into log history and saw that the oxygen was serviced in ZZZ1 on [date]. Three days prior to our flight. This airplane was flying around in this condition for three days and nobody caught it?The reason I am filling this report is that I fear something nefarious happened or the mechanic that serviced the oxygen bottle is incompetent. In either case this needs to be looked into.And our pilots need to be re-educated on how to test the oxygen system. I don't know how many segments this airplane flew with a half shut oxygen valve over a three day period?
Captain discovered during preflight that the oxygen masks did not work properly; resulting in a maintenance action.