When I was doing this, we were shipping station spares. Not all stations had the facility to re-fill O2 cyls and the logistics of having to send them out for filling plus certifying them as serviceable were not feasible. Main bases would handle the re-fill function, stations would get them shipped to them, often OPR or AOG.
Shipping is not onerous, just need the proper shipping container plus dangerous goods paperwork which after a while, I could fill out in my sleep.
Then there were the times we used to render T/X oxygen generators harmless by firing the cap and putting them in a fridge until they were expended.