According to the mastermind behind Adventure Time (Pendleton Ward), Finn and Jake's so called land of "Ooo" is actually.... POST APOCALYPTIC! Thats right, a huge nuclear conflict, named "The Great Mushroom War", caused horrible destruction to the land of Ooo, and many many years later (no one knows exactly), life has begun anew. I'm not sure if this explains all the oddities around the world like vampires, wiener dog princesses, and magical dogs, but it sure as hell explains the very first scene you lay your eyes on in the show's intro :
That's right, a wasteland, filled with.. whats that? Undetonated bombs! There are many other clues hidden throughout the show as well. See if you can find them!
I am sorry if I made Adventure Time a tad bit depressing for you. Penn Ward has actually hinted at revealing this information in later season 2 episodes. I'm sure he will find a way to make it awesome and not disturbing! Thanks for reading!