That's the Option Key symbol (the Shift is indeed an up-arrow), so try option+F12 again.
However... in System Preferences > Keyboard & Mouse > Keyboard, there's a checkbox that determines whether the F-keys control system functions (display brightness, speaker volume, etc.) or the functions of the currently active application. If the checkbox is set to control system functions, you have to hold down the Function (fn) key in order to use F-keys to control application functions, so in that case, you should be able to invoke Quick Preferences in Opera by hitting fn+option+F12.