You won't find Opera in Debian - it's not OSS but you can get a .deb from their site http://www.opera.com/download/
(Get the QT static deb )
and install it like so:
su
<password>
dpkg -i opera-static_7.51-20040602.1-qt_en_i386.deb

Then you want to put it in the menu
update-menus -v

Restart your window manager and it should be there.