Oxygen is definitely still monetized. Think about the canisters or the tanks that people use to go into high altitude environments. Hell, even in Denver or Aspen or Telluride you can go to an 'oxygen bar' for absurd prices. Hospitals will put you on oxygen and then bill you thousands for the service.

Beyond that, I have no doubt that there are some people who would willingly charge everyone for oxygen, if there was only a way. This would be driven by the idea that even though there is plenty in the atmosphere, a person only needs to be deprived of it for a little while before the situation becomes dire.