Nice article about OK/Cancel Buttons. I was researching this because I had to make a decision on Marketo whether or not to put the OK before the Cancel or vice-versa. Apparently, as this study shows, MacOS X and Windows do things very differently. Should the OK come first or last? This is definetely an idiom that people get used to, but doesn’t really matter all that much. In fact, many web applications just do it however they want.
In general, I think that you want the user to click OK or Yes or an affirmative answer more than you want them to click a negative answer. Therefore, using Fitt’s Law, you would try to make the affirmative button closer and bigger. One way of doing this is to eliminate the cancel button and have an X in the top right. But if you have to have both, the choice comes down to order.
I chose affirmative on the left and negative on the right. Mostly because that is how people say it in english. “Do you want to go potty Matthew? Yes? or No?” No or Yes isn’t the way people talk. Mac people will have to suffer.