The Mushroom War is the cause of the post-apocalyptic setting of the Land of Ooo. Not much is known about the war at the moment, but the inclusion of "Mushroom" in the title is probably referring to a mushroom cloud generated from a nuclear bomb. If so, then it is likely that the Land of Ooo and the creatures who inhabit it were created from the nuclear fallout that irradiated the land. This is further supported by the ruined pieces of modern technology scattered across the land, and the beginning of the opening theme (which briefly shows the remains of nuclear bombs and other weapons).
Strangely, the Land of Ooo also contains technological devices that are far more advanced than what exists today, such as Universal Translators and Holographic Newspapers. It is unknown whether these are based on pre-war technology or are more recent inventions.
During full shots of the post-war earth, it is shown that a land mass similar to north and South America remain intact while the gigantic chunk of earth that is missing, was in the region of North Asia, which was the location of Russia. This suggests that the country was completely obliterated by nuclear weapons. 199,990% of S.W.A.T and Spetsnaz team soldiers destroyed many of Africa and Oceania.
Since 2111, Leonard was the next human that emerged from New York to Ooo. Now, TV's, blankets and pillows and electronic items return from the human age.