oxygen injection?

i'm sure hot rodders or truck guys might've used this in the past before, but it just dawned on me a few days ago. power adders for cars all rely on getting more air into the engine, because more air means more fuel and more fuel means more power. so why use nitrous oxide to introduce more oxygen into the combustion chamber when you can hook up an oxygen canister? it would seem to make more sense to me unless it's unreasonably dangerous or more expensive than nitrous oxide gas. any feedback would be interesting.