What is the Full Form of OK ?

OK - Oll Korrect or Olla Kalla in Greek (means All Correct)

OK means agreed, understood if someone is telling you something and you acknowledgment them.Like in a Conversation 

Person 1 : I am going to schoolPerson 2 : OK

Or, in a situation - Suppose you met with an accident last week and some one asks "How are you" and you answer them.

Like in a Conversation 

Person 1 : How are you now?Person 2 : I am O.K.

We use it in many situation in our daily life, without even thinking that its Full Form may also exist.