Human Factors
The oxygen bottle tap should have been redesigned/eliminated years ago, there should be a self sealing coupling that allows full flow when connected, no in between. Have seen on a 737 Jurrasic where, with tap open, but only slightly, one crew could breathe with his mask, but when both crew tried the crew oxy gauge descended to zero.
The oxygen bottle is common to both pilots and should not have to rely on anybody remebering anything in order to provide full flow.
Ideally, if a well designed self sealing coupling is disconnected the pressure should read zero and produce an EICAS/ECAM warning. If it is connected it can only supply full flow. Dream on......