Some aircraft designers seem to favor the bottle pressure on the gauge, not system pressure. DA comes to mind. This can be a trap for the unwary. The main O2 valve maybe turned of during maintenance. During a system preflight check should include, the main valve checked on, EMG should be selected for at least 5 seconds to ensure you are not hearing residual O2 pressure, only. I am very wary of an aircraft straight out of maintenance.
On the subject of another possible cause of this crash. If there is a need to start checking the passenger manifest, maybe it's time to start profiling passengers before departure.