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@PixCatholica, I did some research and replaced libffmpeg.so with the one I downloaded from https://packages.ubuntu.com/xenial-updates/amd64/chromium-codecs-ffmpeg-extra/download
Download the package, find libffmpeg.so in it and extract it.
Open /usr/lib/x86_64-linux-gnu (this path may vary) click with right mouse button on "opera" folder and "Open as Root". Replace the libffmpeg.so with the new file.
Now you can play videos.
@ilbert sorry, I'm in a wrong topic and my last answer wouldn't work for you. Unless the app installs the libffmpeg.so globally, but now I see your answer before posting this and obviously it doesn't work. Sorry. Keep digging the forums 
@ilbert, sorry man.
I'm with Linux Mint 20 "Ulyana" - Cinnamon (64-bit) and use Opera only for testing my websites.
Elementary is attractive OS, especially if you come from macOS, but it is with many flaws. And about Opera, the best way to go is to abandon it and move to another browser (Chromium), which will push Opera developers to do their job. We must not work for them, they must work for us.
