Our old friend, the OK Button, has gone through a few visual tweaks in the last twenty years of Windows:
There is one constant, however: the use of OK. It's an abbreviated form of Okay. But where did this word, and its convenient two-letter abbreviation, come from? The question is answered in this straight dope column on the etymology of OK:
The etymology of OK was masterfully explained by the distinguished Columbia University professor Allen Walker Read in a series of articles in the journal American Speech in 1963 and 1964.
The letters, not to keep you guessing, stand for "oll korrect." They're the result of a fad for comical abbreviations that flourished in the late 1830s and 1840s.
Read buttressed his arguments with hundreds of citations from newspapers and other documents of the period. As far as I know his work has never been successfully challenged.
The Wikipedia entry on OK offers many other theories on the etymology of the word-- but the timing of the first written appearance seems to support Reed.