What extra air? The engine is only going to suck through the amount of air it needs to run at a given rpm. It may have less resistance and at high engine speeds it may be able to grab a little more air, but for normal driving it will still get all it needs to function and the computer will give it the proper fuel ratio. The stoichiometric fuel-air ratio is 14.7:1 of air to gasoline vapor for optimum combustion. The computer is controlling the fuel injectors to get as close to that as possible and adjusts according to feedback from sensors.
Cold air is more dense than hot air so it contains a tad more oxygen per cubic foot. At speed, I don't know how much difference in temperature there really is. A 70 mph gust of wind under the hood tends to move the heat away quickly. At low speed with the AC on and air passing over both the coils, there is probably a greater difference