Thank you for the hint. It works now if you copy
from
/usr/lib/x86_64-linux-gnu/oxide-qt/
to
/usr/lib/x86_64-linux-gnu/opera
with Ubuntu 14.04 LTS.
Sorry for being so upset. But if you want regular people to use this software then this "workaround" is already too much. Cause Opera is a excellent piece of software and if many people use it, it will be maintained in the future.
However on Ubuntu 14.04 the tab bar doesn't fully expand. There is always a gap if you open a couple of tabs. I can't reproduce this by opening a bunch of tabs. It happens after a while. I have no clue what triggers this. Beside the useless "Menu"-Button, which is much too big and a waste of horizontal space, this narrows the tab bar even more.