If oxygen is removed from a sample of air as iron rust, what happens to the particle pressure of oxygen in the air?

Answer : It increases

Rusting is where oxygen binds to iron and forms iron oxide.

So once iron rusts, there is oxygen, just not in air; it's in the iron oxide.

All reactions are reversible, albeit at different rates (the "irreversible" ones are still reversible, but much slower given that they take so much collision luck and energy.