can you provide a link to the video that doesn't work ? does twitter work for you?
I'm running Zorin linux but what I did was:
install chromium-ffmpeg from snap then
find libffmpeg.so on your system.
in terminal: locate libffmpeg.so
mine produced this:
/snap/chromium-ffmpeg/23/chromium-ffmpeg-103551/chromium-ffmpeg/libffmpeg.so
/snap/chromium-ffmpeg/23/chromium-ffmpeg-104195/chromium-ffmpeg/libffmpeg.so
/snap/chromium-ffmpeg/23/chromium-ffmpeg-95241/chromium-ffmpeg/libffmpeg.so
/snap/chromium-ffmpeg/23/chromium-ffmpeg-98516/chromium-ffmpeg/libffmpeg.so
/usr/lib/x86_64-linux-gnu/opera/libffmpeg.so
backup the opera's libffmpeg.so:
sudo mv /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so.bak
copy on of the libffmpeg.so files over (might be trial and error pick one of the newer ones):
sudo cp /snap/chromium-ffmpeg/23/chromium-ffmpeg-103551/chromium-ffmpeg/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/
restart opera. if it works then you are in luck.