Opera isn't available from the Ubuntu repositories so you'll have to download the .deb package from the Opera download page. Adding the official repositories and installing from there doesn't seem to work at this time.
Download the appropriate .deb package and run the installation from terminal. Here, I'm installing the 64-bit version.
~$ cd Downloads
~$ sudo dpkg -i opera_11.61.1250_amd64.deb
|Opera 11 on Ubuntu 11.10|
Personally, I don't quite like Opera's look and feel. But, it's awesome fast. Perfect for anybody looking for more speed in their browser.
Removing Opera is also a simple command away:
~$ sudo apt-get purge opera
The best part is the Opera repositories get purge together so there's no need to open up Software Source to manually remove them.