An oxygen tank would work fine as a CO2 tank, however you do not want to use a true CO2 tank to air up tires or power tools off of. CO2 is stored as a liquid, and the big CO2 tanks have a pickup tube that allows liquid CO2 to be picked up and come out the valve. You do NOT want this if you are going to be airing up tires or running air tools off of. You can use a regular oxygen tank, with a valve and pressure regulator on it.