You'd get better energy density that way, but I think it's offset by worse efficiency & more damage to the engine. The inert gas fraction (N2, CO2, H2O) provides a useful working fluid to convert combustion heat to force pushing on the piston. Burning pure O2 would mean a lot more heat is needed for the same amount of force. Higher peak temperatures & more available oxygen will tend to create hotspots & knock/pre-detonation, & possibly erosion of the cylinder wall & piston from directly burning the metal.

Using nitrous or cool compressed air keeps most of the benefits while mitigating the problems of using pure O2.

Wikipedia says uncited that N2O produces heat when it decomposes into nitrogen and oxygen, but that's the opposite of other comments saying that cold oxygen is better because it's denser

Compressed - cold - pure oxygen, that is.

Held in a lightweight composite thermic bottle.