B737-700
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness; Training / Qualification
Aircraft Equipment Problem Critical
General Maintenance Action
Procedure; Human Factors; Manuals; Chart Or Publication; Aircraft
While performing our pre-flight duties in the morning we tested all the systems as required by the AOM.   Specifically when testing the Crew Oxygen System; the test was valid and the crew oxygen was deemed to be working fine and the test returned the expected results within parameters.  The Oxygen gauge showed a PSI level of around 1;800 PSI.  We departed for ZZZ1 and arrived uneventfully ahead of our scheduled arrival time.  Our trip was over at that point.  Later in the evening of the same day; I received a phone call from the Captain that took our airplane for the beginning of his trip.  He called to advise that when they in turn did the Oxygen Test; their test showed a large drop in PSI and an invalid test.  When Maintenance looked at the problem; they found that when the same airplane underwent maintenance the evening before in ZZZ; the Crew oxygen bottle was swapped out and Maintenance in ZZZ failed to open the Crew oxygen valve all the way.  Again; our [Crew oxygen preflight] test in the morning showed a perfectly valid test and we continued as planned.  Unfortunately; there may be no remedy on the B737 of avoiding a recurrence of this event as the pilots have no way of checking whether the oxygen valve is fully 'open' or not.  The only way would be to notice a lack of PSI on the oxygen gauge and in our situation it showed full.  The overhead speaker test worked fine as well.
Reporter stated he would never have known there was a problem if the later outbound flight Captain had not informed him of the low Crew Oxygen System pressure. He has not seen any specific length of time for a pilot to hold the 'Press to Test' button on their Preflight Checklist. Usually they do a quick 'Press to Test'; checking the O2 Pressure Gage for any noticeable drop in pressure while also performing the oxygen mask internal speaker test. He believes their Crew O2 bottles are composite type bottles. He plans on holding the 'Press to Test' function on the crew mask box a lot longer than he has in the past.
A Captain who had flown a B737-700 aircraft earlier in the day; was informed by an outbound Captain beginning their trip; that the Crew Oxygen (O2) on the same aircraft had a large drop in oxygen pressure when performing the Crew O2 Preflight Test. Maintenance found the Crew oxygen bottle valve handle was not opened all the way.