How do you write OK: Okay, OK, or O.K.?

I did some research on the proper use of the word “OK” because I always doubt whether I am writing it the correct way.

Is “okay” acceptable?

Does “OK” need periods (as in, O.K.)?

I found two helpful articles on the subject:

OK vs. Okay (Grammarly) OK vs. okay (Grammarist)

It turns out that all three are acceptable (OK, O.K., or okay), but OK (without periods and not spelled out phonetically) is the most common.

So, like a lot of common words in our English language, any of them work. Just b...