It would be no different than using using inside. The only difference is that it might be harder to find a flat surface that would get you or the board dirty.
What Would Happen If I Play The Ouija Board Outside?
Nothing any different than playing with it inside. It's hocus pocus and mumbo jumbo. James randi has a million dollars for you if you can prove it's real ;)
en.wikipedia.org
en.wikipedia.org