If you install Opera on Ubuntu 18.04 (or 16.04) it all works – except for video playback. When you first install Opera (or upgrade Ubuntu) You (or at least I) get this:
For me this came down to the /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so file being at the wrong level.
The story goes that Opera is based off Chromium and so relies on the libffmpeg.so version associated with Chromium. Somehow the version that is included with Opera on Ubuntu 18.04 (and 16.04 as well) is actually the wrong version. As videos depend on it then they don’t work in Opera straight out of the can.
The work round is simple and worked seamlessly for me with both 16.04 and 18.04.
If you are paranoid (like me) rename the existing libffmpeg.so in /usr/lib/x86_64-linux-gnu/opera to something else (aka libffmpeg.so.18.04-orig or whatever you like)
sudo mv /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so.18.04-orig
Then copy over the chromium version to the Opera directory.
sudo cp /usr/lib/chromium-browser/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so
Now videos should work.
BE WARNED: If you fixed this on Ubuntu 16.04 and then upgraded to Ubuntu 18.04 the fix has to be done again as the upgrade tramples over the fix.