In Ohio the law does not state it can't but for the most part it's up to the officer but I think id have a case in court.
In my state (PA) you have to get a vertical plate, but it is legal as long as you get it. You can't mount the normal plate sideways.
In Ohio it isn't illegal via state law. But some local laws state that it must be visible from both left and ride side which would make it illegal in some cities. I would guess as long as you don't live in one of those cities you would likely win in court.