I have struggled with OPera and the Menu in the past. Opera and this Menu are a buggy combination (mostly on Opera's side).
Check your body margins and your CSS. For some reason, Opera applies any body margin you have to the menu, making the submenus appear in weird places.
That's all I can think of at the moment. Check your <body> margins. Try using padding instead of margins and see what happens. That's what I had to do.