You can use oxygen gas for pressurization of oxygen tanks, but it introduces a new set of problems, since you've got to get a stable pressure of oxygen gas inside a container with a huge volume of sub-cooled liquid oxygen, and you have to have a high-pressure tank in there anyways to pre-load pressurized oxygen gas, so that you can get it into the tank fast enough. Also it means you've got reactive oxygen gas all around, so you have to redesign things for that.

However, this kind of oxygen-gas pressurization is exactly what's planned for SpaceX's next-generation oxygen-methane rockets.