Pretty sure I can recall in either Pakistan or India they did not have charging facilities on the airport, our slightly depleted bottle was to be taken off field and at that point the Captain put on a pair of specs and declared it was just acceptably in the green band; after that we carried a spare bottle when going East.
Whilst at Andrews we lost nearly all our B747 O2 due to overnight temperatures and cold seals; the USAF had no problem topping us up on stand; guess this would be the case for military operations; future cold operations we shut the bottles off at their necks and left a flag in the flight deck. The charging point was just inside the cargo door.
Of course the UK build standard was LH charging thread.
If I recall USA built commercial aircraft for USA Operators did not have external O2 charging points.
We kept dedicated tools with our O2 charging rig as a precaution; if we were working under the radome on O2 bottles (extended range business jet) we would have an airline blowing to dilute any O2 leak concentration.