Instructions for installing Opera Stable
https://deb.opera.com/manual.html

If you nonetheless want to add the repository prior to installing Opera, please issue the following:

sudo add-apt-repository ’deb https://deb.opera.com/opera-stable/ stable non-free’
wget -qO- https://deb.opera.com/archive.key | sudo apt-key add –
Note: Do not change the entry in the first line from ”stable” even if you use testing or unstable versions of your distro, otherwise you may receive the warning ”W: Conflicting distribution: deb.opera.com” on issuing apt-get update.
Installing Opera

Once the repository is added you can install Opera stable by issuing the following:
sudo apt-get update
sudo apt-get install opera-stable

An another way to add opera-stable.list
opera-stable_installation