The word 'Okay/OK' has about 35 possible origins.
A word so frequently used in our daily conversations still has elusive origins. Though many can now be considered “false etymologies,” there is still no conclusive proof on the origins of “OK” or “okay.”
Among the most commonly circulated origins is the theory that it comes from “oll korrect,” which was coined during a fad for comical misspellings and abbreviations in 1839.
Other theories include that it comes from “Old Kinderhook”, Martin Van Buren’s nickname, or that it was a borrowed word meaning “it is” by the American pioneers from the Choctaws. You can read more at the source