Hey, I just found that the recent Ubuntu package adobe-flashplugin from the Canonical Partner repository:

deb http://archive.canonical.com/ubuntu $distro partner

has been extended to contain also the Pepper plugin of Flash! It starts with package version 1:20150414 which shipped Flash 17, but it is constantly updated, so at the moment of this writing it has Flash 18.

Please add the plugin location to Opera! It is:
/usr/lib/adobe-flashplugin/libpepflashplayer.so