I GUESS THIS EXPLAINS — O.K.?

H.L. Mencken once described “O.K.” as “the most successful of Americanisms,” an estimation verified by U.S. troops during the Second World War, who reported encountering the phrase all over the world. Of all the scores of theories (and sub-theories) as to the origin of “O.K.,” the most widely heard traces “O.K.” to the “O.K. Club,” a political committee supporting Martin Van Buren’s unsuccessful bid for the Presidency in 1840. The “O.K.,” it is said, was short for “Old Kinderhook,” Van Buren’s nickname because he was from Kinderhook, New York.

It appears that this theory is not so much wrong (the “O.K. Club” certainly existed) as it is incomplete. Chances are good the Van Buren’s partisans would never have named their club “O.K.” had the phrase not already been widely known as an abbreviation of “oll korrect,” a humorous misspelling of “all correct.” American speech in the early 1800s was awash in similar abbreviations, two of which, “N.G.” (“no good”) and “P.D.Q.” (“Pretty Damn Quick”), are still heard today.

Source: Yahoo!Answers