pax oxy comes from a chemical reaction in those containers. there is no pax oxygen-pressure that could drop to zero
because this is a totally different system. there is no oxygen-container. and no gauge. and no oxygen until someone pulls a mask. a string attached pulls/triggers an igniter pin which then burns some king of candle. in this reaction oxygen is set free. no kidding.
see:
i read it on wiki, it must be true crew oxy: classic system with a bottle containing oxy and YES! a gauge that theoretically could drop to zero. gotta be a reeeally nerdy TRE to make up that scenario of crew-oxy SUDDENLY dropping to zero in the middle of a flight. and without crew-oxy u gotta descend. maybe u can spend 30min in FL120, then it's down to FL100.