If it's Earth, one person has said Africa. Another person said Europe. And I think we've seen America in ruins after the Mushroom War and before the formation of Ooo, but we don't really know if the city in "I Remember You" and "Simon and Marcy" is part of Ooo or not... Could be. I'm not watching much Adventure Time lately and my memory's a blur.
There are other theories though, like how the Land of Ooo is actually on another planet - but it should be in the Solar System, because we've seen Mars (unless that Mars is a different planet that look like Solar System's Mars. We've also seen Abraham Lincoln as the King of Mars, but I don't know what to do with that. It can mean our Earth and Mars though).
The AT universe is really messed up in my opinion, making this rather hard to answer. That, and I'm too lazy to rewatch the episodes. Well, I've seen them twice already... Maybe I'm just lazy now.
i believe, based on the continents shape and structure, that it would have to be located on Australia, which has a great amount of diversity in itself already. if you change the formation of it a little and add in New Zealand's mass, you get a fairly closely related shape to Ooo. not only this but it can be noted that Ooo is surround on all sides by sea, which is the case of Australia, being the giant island in the sea it is. of course, this is under the assumption that it's not all one giant landmass (Ooo that is), cause if it was then it'd be all the continents (except maybe the continent that existed where the hole in the planet now is).