The full form of OK is All Correct.
The origin of OK is still disputed. Some say it is originated from a dutch phrase ‘Oll Korrect‘ while other say it is a greek phrase which is very near to the English word ‘All Correct’.
One more interesting fact about ‘OK’ is that, it is second smallest word in English and also the smallest statement one can make. This word does not require any other helping verb to complete.
OK can be used as Adjective, Adverb, Interjection.
Further to this, a blog post from New York Times can help on this.