refilling O2 bottles on the road.

It happens I just did this exercise with the local welder's supply.

For a 250cuft tank, the cost was $28 for the O2 + $10 for a month's tank rental(not charged for a week) + $250 for a tank deposit (refunded on return) + $9.50 for a hazmat fee.

After talking to them, one difference for aviator's O2 is that they actually have a lab test a for purity. So there is a paper trail and a neat certificate saying 99.9% purity. (I didn't ask if the remaining .01% was the hazard.)

Given the physics of liquid O2 and the cleanliness requirements for dealing with 2000psi O2, probably very few welder's bottles would fail the test. Next time I'm there, I'll try to ask if there is anything different about the filing procedure that might affect the odds. But the cost is so low that I can't imagine getting anything else.