As you may know, Opera, like Chrome and Firefox is built on three branches: dev, which is the first to receive new features and the most unstable one, beta, which is for allowing users to test the new features and stable.
The latest unstable (developer) version available is Opera 36 build 36.0.2079.0, which brings a few changes and bug-fixes. For information about it, see the official changelog.
Installation instructions:
To install and test Opera Dev on Ubuntu 15.04 Vivid Vervet, Ubuntu 14.10 Utopic Unicorn, Ubuntu 14.04 Trusty Tahr and derivative systems, like: Linux Mint 17.1 Rebecca, Pinguy OS 14.04, Elementary OS 0.3, Deepin 2014, Peppermint Five, LXLE 14.04 and Linux Lite 2, you need to download the deb package and install it via command-line.
How to install Opera Developer on 32 bit Ubuntu systems and derivatives:
$ sudo apt-get install libnotify-bin gdebi
$ wget http://deb.opera.com/opera/pool/non-free/o/opera-developer/opera-developer_36.0.2079.0_i386.deb -O opera-developer_i386.deb
$ sudo gdebi opera-developer_i386.deb
How to install Opera Developer on 64 bit Ubuntu systems and derivatives:
$ sudo apt-get install libnotify-bin gdebi
$ wget http://deb.opera.com/opera/pool/non-free/o/opera-developer/opera-developer_36.0.2079.0_amd64.deb -O opera-developer_amd64.deb
$ sudo gdebi opera-developer_amd64.deb
Because the Opera deb package gets updated frequently, download the deb for amd64 Ubuntu systems from HERE and install it via gdebi.
Optional, to remove opera-developer, do:
$ sudo apt-get remove opera-developer