O Light is written in the key of C♯ Minor.
According to the Theorytab database, it is the 9th most popular key among Minor keys and the 21st most popular among all keys. Minor keys, along with major keys, are a common choice for popular music. The three most important chords, built off the 1st, 4th and 5th scale degrees are all minor chords (C♯ minor, F♯ minor, and G♯ minor).
See the C♯ Minor Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!