OK is short for Oll Korrect or Olla Kalla in Greek which means All Correct in general english.
OK is used in English which denotes approval, acceptance, assent, or acknowledgment.
Let's look at this conversation:
Person 1 : I am going to a party.
Person 2 : OK . call me when you reach there.
Or, in a situation - Suppose you were ill from sometime and some one asks 'How are you' and you answer them.
Person 1 : How are you now?
Person 2 : I am O.K.
We always use it in our day-to-day life, and many of us didn't know that its Full Form may also exist.
Thanks for reading.