Serif "O" used by the Opera Software company. Since a single letter in raster format cannot be copyrighted, this image is in the public domain.
Opera Software ASA
Cropped from <a rel="nofollow" class="external autonumber" href="http://promote.opera.com/logos/Opera_logo5_t.png">[1]</a> (all copyrightable content removed, only the public domain "O" kept)