Setup key with:

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

Setup repository with:

sudo sh -c 'echo "deb http://deb.opera.com/opera-stable/ stable non-free" >> /etc/apt/sources.list.d/opera.list' 

Setup package with:

sudo aptitude update
sudo aptitude install opera-stable

or you can install one of these packages

Package: opera
Version: 12.16.1860
Package: opera-next
Version: 12.16.1860
Package: opera-stable
Version: 43.0.2442.806
Package: opera-beta
Version: 43.0.2442.686
Package: opera-developer
Version: 44.0.2505.0