here is a little secret the doctors will not tell you....
industrial grade oxygen has a higher standard of purity to meet than hospital grade...
head on down to your local welding supply store, buy a small cylinder, and then you can just exchange them for full ones, kinda like propane..
better quality at about half the price ( of course the cylinders will not be painted very pretty )...
I would be very hesitant to offer that advice, because of the potential of contamination, if not from the tank then from the lubricant and who knows what all else. While the oxygen might be pure, it might not be dry, and the moisture could be serious bad karma.