Believe it or not, there isn't a language specialist on earth who can tell you for sure!
Some Scholars say it came from our 7th president, Andrew Jackson, who wasn't all that well-educated and had a habit of misspelling "all correct" as "orl kurrect." When he was too lazy to write it out, he simply wrote "OK" or so the story goes. However, there are no written examples to support this claim.
Others say it comes from the Jamaican " oh ki" for all right," while President Woodrow Wilson said it came from the Choctaw word " okeh."
However, most linguists trace its origin to presidential election of 1840. Democratic nominee Martin Van Buren was born in Old Kinderhook, NY, and one of the groups that supported him was the Democratic O.K. Club. Other Van Buren boosters began using the letters O.K. to signify their support of the candidate, and when he was elected everyone was assured that the country would be O.K.