Is the mouse a Logitech? If so, then I would recommend updating the drivers, because I've heard of this happening with Logitechs and Opera. If that doesn't apply, you might want to check the following website: http://my.opera.com/
Look for new drivers anyway in case you haven't, and check support forums at Opera. Also mess with the mouse properties in the Control Panel if you haven't already. That's all I've got for ya though, good luck.