I appear to have had the same problem. This may not be the best solution, but I've found this to work so far:
- Install chromium-ffmpeg-codecs-extra (which is currently version 44 as at 2015-08-27)
- Maybe back up /usr/lib/x86_64-linux-gnu/opera/lib/libffmpeg.so.31
- Have permission to write /usr/lib/x86_64-linux-gnu/opera/lib/ and contents (helps to be root or have sudo)
- Symlink /usr/lib/chromium-browser/libs/libffmpeg.so to /usr/lib/x86_64-linux-gnu/opera/lib/libffmpeg.so.31
YMMV, but that's worked for me, for now!