Finally an issue to which I can speak with some authority. I have been commuting with oxygen bottles for 11 years (39 mi RT for 10 years, last year in new location 25 mi RT). What I did was to go to a sheet metal shop and have them build me a frame. Basically just a strip of aluminum with the bottom folded up to make a base for the tank and the top folded down. The top has a hole cut in it to through to accommodate the neck of the bottle. Holes are drilled through the sheet metal holder to allow it to be bolted to the frame and attached with u-clamps to a standard rear rack. One of these on each side of the bike and a spare bottle held to the top of the rack with bungee cords.

It works. Issues are: most rear racks are flimsy, particularly where they attach to the frame under the seat. I had two heavy duty (solid) aluminum rods for this purpose. The rig is bit noisy (people will hear you coming). You'll need to inspect the mounting from time to time. The bolts shear off about once an year! And 3 oxygen bottles are heavy, so you'll need to buy yourself a Suley cross-check to carry it all.

Finally, remember that the cannula goes outside your earphones and helmet, or you'll rip your head off when you go down.