This is what worked for me in Debian 10:
[*]Install chromium-ffmpeg snap:
$ sudo apt update
$ sudo apt install snapd
$ sudo snap install core
$ sudo snap install chromium-ffmpeg
[*]Keep the original libffmpeg.so file in /usr/lib/x86_64-linux-gnu/opera/ and create inside there a directory lib_extra:
$ sudo mkdir /usr/lib/x86_64-linux-gnu/opera/lib_extra
[*]Create a symlink in the lib_extra directory of the snap latest version libffmpeg.so:
$ sudo ln -s /snap/chromium-ffmpeg/current/chromium-ffmpeg-98516/chromium-ffmpeg/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/lib_extra
Note: the latest version in my case is 98516, make sure to check yours inside directory /snap/chromium-ffmpeg/current/
After these steps Opera browser plays all kinds of videos i've come across.
Thanks to @mr-matt-eastwood and this post that provided the solution steps!