Should Ok Be Capitalized?

It's very simple: you have two choices if you want to say something is okay. You can spell out the word in full (okay), or you can use capital letters (OK). What's unacceptable is to write 'ok' in lower case.

Should OK be in capitals?

The generally accepted form is 'OK' – upper case, with no full stops.

What is the correct way to write OK?

According to Gregg Reference Manual, okay is written without periods. “In sentences, the forms okay, okayed, and okaying look better than OK, OK'd, and OK'ing, but the latter forms may be used.” When used as a verb, spelling okay out is preferred. OK is the form supported by Merriam-Webster and the AP style guide.