I choose “okay” every time, because it’s the only logical and acceptable choice. It’s the only one that represents an actual word.
As an abbreviation, O.K. is not the shortened version of any two modern-day words. It’s origin is uncertain at best and probably the result of an error made long ago. The two letters, along with the two marks of punctuation, mean nothing.
I don’t use abbreviations that don’t represent actual words because I am not a crazy person.
On the other hand, the unpunctuated version of OK does stand for something:
Using it in place of anything but our 46th state makes no sense.
So I choose “okay” when attempting to express that assent or agreement. It’s the only sane choice.