I use "okay," and nothing else -- and, as a Professional Grammar Nerd and copy editor, it'd be my instinct to change anything else. "OK" stands out too much on the page for me. Plus, it's the postal code for the state of Oklahoma.
But I've definitely seen variations on "okay" in published books. I swear I saw either "OK" or "O.K." (argh!) used throughout a YA novel I read recently, and it drove me nuts. It all comes down to the publisher's house style.
I think "okay" is the most correct.