Ref to Hogg and B2N2, Whilst O2 bottle might have an over-pressure relief (overboard) it would need the likes of a fire to cause bottle pressure to reach that relief level; sub zero conditions might cause seals to leak and I had this once with a B747 classic (bottle O2 stowage in forward hold) where it almost lost all its O2, only luckilly at a USAF station and they had plenty. As per Sepp's schematic, the aircraft had remote fill point, if the bottle is shut off you cannot fill. British built aircraft had left hand thread fill union (not aware of Euro etc) USA built had right hand; some Countries / airports do not allow on-board O2 re-charging; in fact in Asia there are some major airports which do not allow O2 on base and I have had to remove bottles and have them taken off airport. It is possible, that a long layover procedure might be to close O2 bottle, but this is not standard and likely a maintenance task; if left closed then the pre-departure check of masks would quickly dispel the manifold pressure and the bottle would read zero!