No one knows for sure. all of the other theories mentioned on here are accceppted by many lingusits. another leading theory that isn't mentioned here has to do with the 8th president of the US, Martin Van Buren. Van Buren, from the town of Kinderhook, New York, formed a political party during his presidential run in 1840 called 'Old Kinderhook'. The club became know as OK, thus propagating the usage of 'OK'. However, many linguists point out that the usage of 'OK' predates(was used before) the 'OK' club, it was just the popularity of Martin Van Buren that made the phase 'OK' famous.