Well, there's
this repository. You should download the highest version available with the prefix
chromium-codecs-ffmpeg_, paying attention to the architecture in the ending just before the extension:
_amd64 or
_i386.
Once you have the right package, you should extract the file libffmpeg.so and put it inside /usr/lib64/opera/lib_extra (usually this lib_extra folder doesn't exist, so you have to create it).
There's also a Slackbuild for this:
https://slackbuilds.org/repository/14.2 ... eg-codecs/
The strange thing is that Opera already comes with libffmpeg inside /usr/lib64/opera/ but it seems that this lib doesn't work even putting it inside /usr/lib64/opera/lib_extra
I hope this can help you.
EDITED: it turns out that the current Ubuntu repository doesn't have a lib compatible with Opera current version (built on Chromium version god-knows-which), so maybe you can use this unknown but working repository:
http://repo.herecura.eu/herecura/x86_64/ . Just check for
opera-ffmpeg-codecs- prefix.