User blog comment:*floofcake*/Theory Time 1: Is Ooo Earth?/@comment-5927259-20140112063610

In "Simon & Marcy," there's a flashback from 996 years ago, the nuclear ended maybe just a couple years previous to it. It is very obviously this is Earth, Simon and Marceline travel across a city.

In "Finn the Human," it shows what happened to the world if the Mushroom Bomb wasn't detonated. You can see Simon's crown freezing the Earth, it does not look like Ooo at all and there are no mutations such as Candy people and there are humans.

So yes, it takes place on Earth.