The word OK is reported to have arisen during the American Civil War, when the armies wrote in a blackboard, at the end of a battle day, the casualties they had suffered. It was written in the following way: [number of people] K (K for "killed"). Therefore, OK means "0 [people] killed", undoubtedly a synonym of a good day, or a controlled situation. That could be one of the things, it seems that OK started way before that. OK means something like this: 1839, only survivor of a slang fad in Boston and New York c.1838-9 for abbreviations of common phrases with deliberate, jocular misspellings (cf. K.G. for "no go," as if spelled "know go"); in this case, "oll korrect." Further popularized by use as an election slogan by the O.K. Club, New York boosters of Democratic president Martin Van Buren's 1840 re-election bid, in allusion to his nickname Old Kinderhook, from his birth in the N.Y. village of Kinderhook. Van Buren lost, the word stuck, in part because it filled a need for a quick way to write an approval on a document, bill, etc. The noun is first attested 1841; the verb 1888. Spelled out as okeh, 1919, by Woodrow Wilson, on assumption that it represented Choctaw okeh "it is so" (a theory which lacks historical documentation); this was ousted quickly by okay after the appearance of that form in 1929. Okey-doke is student slang first attested 1932. Greek immigrants to America who returned home early 20c. having picked up U.S. speech mannerisms were known in Greece as okay-boys, among other things. Read more: http://wiki.answers.com/Q/What_is_the_origin_of_the_word_%27Ok%27#ixzz1Mk9TD0nX

From the linguistic and historical evidence, that it comes from the Scots expression och aye, the Greek ola kala ('it is good'), the Choctaw Indian oke or okeh ('it is so'), the French aux Cayes ('from Cayes', a port in Haiti with a reputation for good rum) or au quai ('to the quay', as supposedly used by French-speaking dockers), or the initials of a railway freight agent called Obediah Kelly who is said to have written them on documents he had checked. http://oxforddictionaries.com/page/originok