How do you remove oil paint over spray on a car?

You might try a weak reducer like a urethane reducer, Lacquer thinner is probably too strong and may damage your paint. It might work if you're quick. Some reducer on a clean white cotton rag, and another clean rag to take it off. Depends on what the paint is that the over-spray came from. Next choice is going to be wet sand with 2000 sand paper, and run a buffer over it! At least your car will look real nice after all that work!