I don’t think much oxygen is occupying the empty part of the tank. Typically you’d need an incendiary to light them up.
What fills the empty space as they drain the fuel from the tank?
Air.
These tanks are made from a very heavy steel with the expectation nobody would actually SHOOT one of them.
But you can darn near count on that being an ingredient of the next attack.
.50 cal into a darn-near empty fuel tank.
Typically with such tanks the vapor space is a nitrogen pad at a few inches water column pressure.