Sergey Kabashnyuk

1. Add source

   deb http://deb.opera.com/opera/ etch non-free

2. Add key

   wget -O - http://deb.opera.com/archive.key | sudo apt-key add -

3. (If you get "The following packages cannot be authenticated" warnings, you'll also need this:)

   sudo apt-get install debian-archive-keyring

More links:

OperaBrowser ubuntu wiki

Opera official site