It can be inferred that the Land of Ooo is an isolated chunk of the western United States, including Los Angeles and San Francisco. The following evidence supports this (credit to BabyEatingFox for some of these points):
Signs for Interstate Route 101, the main road passing up the coast of California, are seen in I Remember You:
Several shots from the episode Simon & Marcy resemble locations in Los Angeles, such as the Vincent Thomas Bridge (left) and the LA River (right):
In Joshua & Margaret Investigations, the investigation bureau appears to be in the ruins of San Francisco. The bridge in the background of the right image closely resembles the Golden Gate Bridge:
In Bonnibel Bubblegum, there is an Arizona license plate on the wall. Arizona borders California:
The show was written in California, so it makes sense that the writers would use their actual location as a base.
Despite this, it is important to keep in mind that the location of Ooo is kept deliberately ambiguous, although it is almost certainly in the United States.
I have also attempted to determine the size of the Land of Ooo. You can read the results here. TL;DR: It's about 450 kilometres across according to that set of calculations.