I have found this fossil tooth, could it be pliosaur ? Or maybe theropod?
Most likely plesiosaur. That fluted structure is indicative of marine reptile teeth. My first guess was mosasaur but their are a lot of plesiosaurs from that formation so I would go with that. It is obviously a shed tooth which makes it harder. shark teeth flare out at the base becasue they attach differently than marine reptile teeth, sharks need more surface area for a strong connection.  Hopefully a marine reptile person can give you a tighter identification, I mostly work on dinosaurs.