Jakob Nielsen's latest Alertbox recommends that order of OK vs Cancel should be determined by the standard order defined by the platform's interface guidelines. Alas, if you're designing for the web, order becomes problematic because Windows interface guidelines put OK first, whereas Apple guidelines put it last. Nielsen suggests that you base the order on the installed platform of your audience, i.e. if you have predominantly Windows users, you might be best served by putting OK on the left.
The best part of this essay is that he provides persuasive arguments for either side, making it easy to see how user interface designers can pick a side and use the argument to justify a decision. But the main argument here is to use what your users are accustomed to, and when you can, write more meaningful labels than "OK" for your confirmation button.
As web designers we challenge the notion that how we do form interfaces needs to follow interface guidelines strictly. If we're smart, we stay aware of these guidelines and the Human Factors research that has gone into designing interface rules followed by traditional software designers. But, most of the time that we're dealing with buttons, we're not dealing with dialogs with only OK/Cancel buttons. Often we're designing lengthier forms.
So what happens if we look at our user statistics and decide that what our users are accustomed to sucks? We might feel justified to think about form design and how it applies to the web and go against the conventional rules for Windows and Apple interfaces. In the case of dealing with OK/Cancel buttons, for example, many interface designers opt to treat Cancel as a link rather than as a button. This way, we end up minimizing the Cancel button and encouraging the user to gravitate towards the submission button.
If you're interested in how to tackle forms for the web, Luke Wroblewski deals with issues like this and more in his excellent book Web Form Design: Filling in the blanks.