My general sense has been that the world of pulp and/or newsprint (specifically comic books, newspaper comic strips, newspaper political cartoons, and newspaper stories in general) prefers to use "OK." It saves space and ink.
But off in the world of formal literary prose and book publishing, the word "okay" is preferred because it looks like and feels like a real word. It therefore flows more nicely through the retinas than a clunky and capitalized abbreviation would.
(The in-between world of journals seems to be an in-between mix of the two.)
At least that's my understanding.
Meanwhile, I am proofing a non-fiction book for a guy who already has the book deal signed. He just has to deliver the completed manuscript by July 1st to the publisher. This guy is a New York resident with a background in newspaper journalism, and so he is partial to the conventions of newspapers. Additionally, he's also very biased toward the universe-shaping maxims uttered by that pantheon of immortals who flit around inside the illustrious halls of the almighty New York Times. So his whole manuscript is full of "OK." But it's been very jarring for me to read that dispicable abbreviation. So I checked the internet, then I came here to ask you guys. Both the internet and you guys said: "No difference. Go wit whatevuh."
After asking you guys, I wrote him a note saying I personally detest "OK," and then I explained why. Then I said it's ultimately his choice.