In casual use, OK is acceptable as an interjection. " In fact, you really shoudl spell it out in most instances. Ok is really not used.
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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.
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.