There are a few ways to get Opera installed in Ubuntu, but with any Debian based distro you’re best bet is through apt-get. Unfortunately the default sources in Ubuntu do not include Opera, so you’ll have to jump through a few hoops to get it up and running.
First open up your sources file in your favourite editor:
sudo pico /etc/apt/sources.list
Then add the following source:
deb http://deb.opera.com/opera/ stable non-free
Next you’ll need to add the GPG Key:
wget -O - http://deb.opera.com/archive.key sudo apt-key add -
Now you can install Opera using apt:
sudo apt-get update sudo apt-get install opera
In the event that you get The following packages cannot be authenticated as a warning, you will also need to run this command:
sudo apt-get install debian-archive-keyring
From there just run the install command again, and you should have Opera up and running in a few minutes.
If you run into any issues, or need help installing any plugins, I would recommend checking out the Community Documentation.