I tried to follow the instructions on the opera page, but it didn't work. I was assuming the base install should be the same for 64-bit, but it doesn't look so. After executing

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

successfully

sudo apt-get install opera

says

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package opera is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'opera' has no installation candidate

Below on the page I see

A 64 bit Linux version is available, you should get it automatically either from the repositories or from Opera

I haven't tried to download it manually, since I prefer to get it from a repo. It it possible?