Should yes/no dialogs also have a cancel?

1. What will happen if you choose OK instead of cancel in this dialog box?
2. Should cancel be on left or right?
3. Is yes on the left or right?

What will happen if you choose OK instead of cancel in this dialog box?

Listing OK last improves the flow, because the dialog box "ends" with its conclusion. Also, as with Previous/Next, you could argue that OK is the choice that moves the user forward, whereas Cancel moves the user back.

Should cancel be on left or right?

In most languages, people are accustomed to read from left side to right side, so it accords with our speech habits to put ok-key on the left side of cancel-key. 2. “Ok/cancel” type make it easier to see and select ok-key.

Is yes on the left or right?

Yes - Right; No - Left.