When a gas heats up, the pressure goes up. With race cars, changing pressure is a bad thing because you can set up your car on tires with say 28.7 psi, if you are racing with oxygen, the pressure can jump up by a lot, throwing off your car's set up and you won't have enough grip going into tight corners and you'll understeer causing everyone else to go in underneath you.
I used to run Nitrogen in my kart tires, big difference.