We at LHR
on our aircraft use a (pure) oxygen recharge cart, which is attatched to the charging point (optional fit) on the aircraft via a hose and uses differential pressure between the aircraft and the cart to charge the oxygen cylinders on the aircraft. This is done when the O2 pressure on the aircraft drops below acceptable levels, under very strict procedures. The only compressed air is the oxygen itself.
I understand that this maintenance practice is not permitted in the US and elsewhere for safety reasons.