Never heard of a plasma cutter that uses O2. You would have to have tanks the size of railroad cars. My Lincoln Procut 55 uses air at the rate of 12 cfm at 60 psi.
As to your other question. The change in metalurgy while cutting with air/plasma is so minute it is not even in the equation. However, using shop air in your mig is not recommended. It will cause degredation of the welds.