From the way I understand it, the libffmpeg.so file that Opera uses doesn't contain the proper codecs for certain videos. This is how I fixed it for KDE Neon (Ubuntu variant). Make sure you have the Chromium browser because this will break Opera in the process and you will need the Chromium libffmpeg.so file. Use this at your own risk.
sudo updatebd (update locate database if you never used it before)
sudo locate libffmpeg.so (shows location of all libffmpeg.so files)
sudo rm /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so (removes the libffmpeg.so that Opera is using, if your results are different from the locate command then use that path)
sudo cp /usr/lib/chromium-browser/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/ (copy the Chromium libffmpeg.so into the Opera directory)