Did you know that OK stands for Old Kinderhook?
OK was popularised and begun in 1840 as a slogan of the O.K. Club, a Democratic party political club of the 1840s. It is the nickname of President Martin Van Buren, born at Kinderhook, New York. The member of this club was known as "an OK guy" because the party was seen as a positive thing at the time. Later on, it is used to mean someone who was "alright!"
So why are we seeing okay written? ‘Okay’ is the form used when there is an inflectional ending: okayed; okaying. It is called the quasi-phonetic written form like deejay for DJ, or emcee for MC, or jaycee for member of Junior Chamber of Commerce (in American English). Unless there is an inflectional requirement, we should use OK in written communications.
Reference: Future Perfect
Photo: Everything is OK.