To install Opera 12 on Ubuntu 12.04 you'll need to add the official repository. There's no Opera PPA available on Launchpad at this time.
~$ sudo add-apt-repository 'deb http://deb.opera.com/opera/ stable non-free'
~$ wget -O - http://deb.opera.com/archive.key | sudo apt-key add -
That should do it. The rest is a simple update and install away.
~$ sudo apt-get update
~$ sudo apt-get install opera
Just sit back and wait for the download and install to complete.
For some reason, I could update my source.list from Opera's repository. If that happens, just download the install package from the Opera download page. Package repositories sometimes don't work as they should.
|Opera 12 on Ubuntu 12.04 64-bit|