Actually Opera 8.0, unlike 7.5x and earlier versions, does not appear to use an
OS-native multi-document interface (MDI) even if you do use the "Windows Native" skin.
Compare these screenshots of Opera 7.54 and 8.0 (both with the "Windows Native" theme). Opera 7.54 has a [tt]Window[/tt] menu as well as window controls on the menu bar corresponding with a maximized MDI page, while Opera 8.0 does not.
Likewise, Opera 7.54's OS-native (in this case Windows-native) MDI allowed you to cascade or tile individual windows, as seen here. To the best of my knowledge this capability is not present in Opera 8.0.
That said, although the MDI of Opera 7.54 and earlier was a bit more versatile, I rarely saw the need for such capabilities in a web browser (rather preferring the simpler tabbed interfaces of Mozilla and its derivatives) and Opera 8.0 may be easier to use.
UserAgent: Mozilla/5.0 (Windows; U; Win95; en-US; rv:1.7.7) Gecko/20050414 Firefox/1.0.3