Try running opera with the following line:
opera --debugfont | grep -A 25 'Xft baseline'
and close it immediately. See the grep output in a console. Most probably lcdfilter is set to legacy. It this is the case, do the following:
$ echo 'Xft.lcdfilter: lcddefault' >> ~/.Xresources
2. Restart your X session.
3. See obvious changes in opera's rendering
This overrides default Xft lcd filtering engine (see http://spasche.net/files/lcdfiltering/ for details).