Normally, when you click the OK button, you are either acknowledging a situation or a problem (as in an alert box) or are giving your consent to an operation being executed.
It was the case in earlier versions of the Macintosh system software and the Mac OS that the OK button was the default button except for in cases where clicking OK would result in data damage. This rule has been followed less and less rigidly, and in Mac OS X (as an example), the OK button is the default button even when it comes to emptying the Trash.
The OK button is universally phrased except for in a few languages that do not use a Western alphabet. As a result, it is very consistent acknowledgement button.