Add the repo
sudo sh -c 'echo "deb http://deb.opera.com/opera/ stable non-free" >> /etc/apt/sources.list.d/opera.list'
Install the key:
sudo sh -c 'wget -O - http://deb.opera.com/archive.key | apt-key add -'
Update:
sudo apt-get update
Install Opera:
sudo apt-get install opera