Oil is almost always used when cooking vegetables or meat at high temperatures in the oven, but I'm not sure what it actually does - it obviously provides flavour, but there seem to be other purposes.
Surface water prevents the maillard reaction from occurring because the temperature cannot exceed 100C. I presume that oil does not have this same property? How does oil aid in crisping the surface? Does it do anything else? Does it affect the form or speed of heat transfer?