You probably typed this because you were bored and therefore clicked on the first link on your search engine which is how you are reading this and you probably know what it means, If not, then continue reading below :)
It means to be suprised, like this ---> O.O <--- (eyes open wide)
Or to be looking at someone in a "one eyebrow up" kind of way like this ---> O.o <--- (It may be flipped to this ---> o.O <--- although It does means same thing) it's a way of saying "whaaat?" or "I don't know what you're talking about" or even "don't look at me, I don't know!"
Or it can mean "I'm confused and don't understand the current situation" like this ---> o.o <--- (eyes wide open but sort of small and hidden) this may be due to confusion or maybe simply trying to look cute.
Hope this helped...Everyone has their own opinion of what it means :D <---Happy face haha :D