Yesterday it was rainy, so I turned on the A/C to de-fog the windows. After a minute, there's this scent of motor oil, so I turned off the A/C and the scent goes away. Logically it has something to do with the air conditioner.

What's the diagnosis?

 

Do you have an oil leak? Oil might be leaking and dripping onto the exhaust manifold causing the burning oil smell and when you have ac on your drawing the smell into the cabin.

 

Turning on the defroster changes the vent setting from recirculation to drawing in outside air. Were you driving behind a car or truck with a strong exhaust smell?

 

Are you sure it is not exhaust smell, or as others said burning oil off a hot surface.

Open the hood and see if have any top oil leaks, then look for exhaust leaks.