Usually 'okay', sometimes 'ok' for whenever I feel like writing it shorter (like being busy in game). I use 'okey' and 'k' exclusively for being a doofus.
I usually use ok for random stuff, okay if I'm feeling more "mature" (lol), k for being lazy, okie for being weird, and kay for slangish but still I like how it looks way. (if that makes sense)