Because he moves his tail so much without meaning to, Ojiro has accidentally hit his classmates or smacked them in the face with his tail on multiple occasions,, he apologizes profusely as they tell him that it's ok, and he's a bit guilty about it the rest of the day and is more self-conscious and careful with his tail. He mostly hits Kaminari with it, because his tail will jerk or wag and Kaminari will be hit in the face while he's playing with it. Ojiro doesn't particularly mind Kami playing with his tail, though. He's used to it.