But...isn't it true that you can get filled CO2 tanks, use 'em up, refill them all you like, without any kind of license?
I mean, why would it HAVE to be oxygen (a known chemical corrosive?) Heck, put all them little germs in some nice frosty carbon dioxide. Keep 'em all snug and happy 'till you want to do your terrorist jazz...(sorry for the sarcasm)
Not buying it.