Up to version 12, Opera made their own engine for Opera, called Presto. But 2013, they decided to re-create the browser from scratch, built on Chromium with the Blink engine. The first version of the "new Opera" was Opera 15, and the "old Opera" was discontinued. But as the new browser was built from scratch, it lost many features. Therefore, there are still many users that want to use Opera 12. As it differs very much, there's a second wiki page for that version. You can access it here: OperaBrowser12.
The best and easiest way to install Opera is by just downloading the deb-package from Opera's website. When you install the package, it will add their repository, automatically configured for updates.
When you have downloaded the file, you can install it by double clicking on the file to open Ubuntu software center. If you don't want to add the repository, you can install the package by opening a terminal in the directory where you have downloaded Opera, and run this command:
sudo dpkg -i <opera-package-name>
When you do this, you will be asked if you want to add their repository or not.
You can read more about this here: http://deb.opera.com/manual.html
You can convert the deb package with this script: https://gist.github.com/ruario/8416e36372f1a976a713
Install this package to get flash:
sudo apt-get install pepperflashplugin-nonfree
Java is not supported currently, as they only provide a NPAPI plugin (which are not supported on both Opera and Chrome on Linux).
Get H.264 working
In order to get support for the video format H.264, you have to install ffmpeg.
In Ubuntu 15.04, ffmpeg should be available by default. If you not already have it installed, run this command:
sudo apt-get install ffmpeg
Note! There's a bug in ffmpeg 2.5.* that causes Opera to crash. With the above command you will get that version, even though the latest version is 2.7.1. Until the package gets updated, it's recommended to compile ffmpeg yourself (see instructions further down).
14.10 and under
In Ubuntu 14.10 and under, ffmpeg is not in the official repositories. One way of getting it is to add a repository:
First, make sure the ffmpeg-real package isn't installed, because they will conflict otherwise:
sudo apt-get remove ffmpeg-real
sudo add-apt-repository ppa:kirillshkrogalev/ffmpeg-next sudo apt-get update sudo apt-get install ffmpeg
If this doesn't work, or if you want to install a newer version of ffmpeg, it's recommended to compile ffmpeg yourself. See instructions below.
If you want to compile ffmpeg, this guide is highly recommended: http://ruario.ghost.io/2014/12/19/adding-h-264-video-support-to-opera-for-linux/.