I don't have an option to undecorate (permanently) in Ubuntu distros like ZORIN OS 11 and LINUX MINT, if I remember right Kubuntu didn't have it neither. I broke few Ubuntu distros installations trying to learn to hide titlebar in Opera similar as Chromium/Chrome has it (and also Firefox with addons like 'No caption' and 'Hide Caption Titlebar Plus')
I did this because Firefox seems to slow to me even on fast computer, Chrome has it's own disadvantages and other browsers don't have capability to add plugins or are not updated or similar...
So this leaves me with Opera-unfortunatelly it doesn't have a nice upper side because of the titlebar and this also wastes space; I am used to this being improved like in Chrome or Firefox.
There is a software COMPIZ which undecorates all windows in Ubuntu distro (Email client, file explorer, Opera and everything else), but then you cannot even minimize or exit/close the programs in a nice way which is even worse.
I read online that some of other Opera users want this feature too.
I would even be willing to make a small donation if this would be done.