What is the easiest owl city song on piano?

The easiest Owl City song on the piano is probably "Fireflies". The song is made mostly of the notes C, D, F, and G and the chords that can be derived from it. As an introduction try learning to move between C, G, , Am, and F major 7 (Fmaj7). There isn't much you won't be able to learn or at least adapt once you get the hang of those.