So, I go out walking a lot to town and back. Every single time this guy in this car passes by, he yells 'OY!' at me really loudly for no reason. Always at me, no-one else. I do not know this person but I want to smash his headlights. Why would he do this? It is really infuriating. Why can't he just leave me alone?
How old do you think he is? If he's under about 20 years, it could be simply childish antics.
Is it possible it is someone you know and haven't been recognizing to be someone you know?
My first guess is he's interested in you, and doesn't know how truly obnoxious that behavior is.
My second guess is he's getting a thrill out of scaring you.
Since it has happened numerous times, and you note it is every time he sees you, I think I'd be inclined to take a photo of the car he's in (you didn't mention if it is the same one each time) if I could get one discretely -- without him seeing. I would give a copy of that photo and written description to someone, just in case anything happened -- and if he ever approached in a threatening manner I would tell him if anything happens to you, the police will have evidence he's been harassing you, along with a written description about his behavior toward you.