This doesn't work. First of all, you'd still have the same mass of CO2, so the insulating effect wouldn't change. Those molecules would still be there to interfere with infrared radiation trying to escape into space.

Second of all, you're talking about gargantuan quantities of oxygen (or nitrogen, or whatever). CO2 is roughly 1/2500 of the atmosphere, so to achieve a given amount of dilution, you can remove one CO2 molecule or add 2500 other molecules.

Finally, I don't know what the side effects of increasing global air pressure would be, but I'm sure there would be significant effects!