Menu bars are very standard and all over software. While traditionally menu bars offer access to all app’s functions most of those are used in faster and more convenient ways (toolbar buttons, hotkeys, context menus).
Standard menu control is also inflexible and can waste good chunk of space on maximized windows.
Opera has native hotkey (Alt+F11) for enabling and disabling menu bar. For some reason by default that hotkey is marked as Mac and Linux specific.
This can be easily edited out (see post on Opera hotkeys for details) and will work on Windows just as well.
Using interface button
Since toggling menu in and out all the time is hardly best interface concept it is more viable to replace it with another control that will serve same purpose but without flaws.
It may seem like a small trick but considering that browser is where we spent most of time nowadays (or at least Wakoopa stats say so) such small tweaks ensure great usability and plenty of seconds saved over time.