The "O" is because the buttons on the virtual Pico-8 console are O/X, which are mapped to Z/X, C/V, and N/M when playing on a computer. It is a bit confusing outside of Pico-8 (like here in an embedded web player).
Indeed. This is why I always mix it up; I'm not choosing or thinking about them especially. :P I'm pretty much determined that next time I'll just put the letters in the game, though, or perhaps both. It annoys Pico-8 folks but that seems worth it in order to make it so other people can figure out how to play!