Usually okay, sometimes ok. Never o.k and alright is a never ever for me as I was always told it is not even a word and that it is supposed to be all right (sorry, no offense to anyone there)
You're right, Elaine. "Alright" is not a word. It's "all right." I use ok or OK because I'm lazy and I don't like to use periods much except at the end of sentences. I also write "Dr C," "Mrs G," etc.