OK.... Ok.... Okay.
Which is the correct one ?

Thank you for your patience.

In casual use, OK is acceptable as an interjection. "OK, fine, we'll do it your way."

Okay is better, and is the only way to write it if you mean the verb "to approve" or the noun meaning "approval" in contexts like "Please reivew this and give me your okay on the changes." or "You need to okay this before it goes to the printer." That also applies to the ajective meaning "accpetable," as in "The movie was okay, but the book was better." In fact, you really shoudl spell it out in most instances.

Ok is really not used.

It's fine to use OK or okay.

Don't write ok or Ok - even though the device you're typing this on will often give you those as options to click on.

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OK, I understand.

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Actually, this is wrong. OK is the original, correct spelling. Okay is a bastardization of the word that people started using because OK didn't look like a word to them. Always use OK; never use okay.

 Rover_KE's reply was promoted to an answer.