if the plane was used by american airlines, it might be by virtue of being with usairways and the merger has the lineage of the plane confused.
I seem to recall, but cannot be sure, that if the oxygen bottle is mainly full, you can top it off in place.
but if it is completely empty it has to be removed to refill
but it has been awhile, someone else might want to check.
the dc9 series had the bottles right in the cockpit, but not the 737.