Cold air = DENSER air, not more oxygen. It just means that the air to fuel ratio goes up ever so slightly, giving a denser combustion area.
and denser air contains more oxygen becuase molecules are packed together. 
Parp, wrong again.
Denser air = more water molecules in the air, thus making it denser, it is only when these are heated that a smidge of oxygen is released in bond breaking.
Anyhow, this is all besides the point, engines will perform better warm, also due to warm air having better explosive properties. (think about it.... it's why boilers etc all have preheaters)