When writing fiction, should I use "okay" or "ok"?

Usually if you use the letters OK you capitalize them. In the novel I'm currently finishing up, I have repeatedly found myself typing OK, then replacing it with “okay.” Why? To me, OK seems to stand out and feel slangier, while “okay” feels like a normal word and doesn't call attention to itself, unless you have Cleopatra saying okay to Julius Caesar, in which case it feels like a total anachronism. Even though okay has spread throughout the world, it still feels like slang and, particularly, American slang from the 1830s.

But if it's okay for your characters or narrator to say OK at all, you can spell it as you wish; but your copy editor will appreciate consistency.