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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.
@amgpuma right click -> Open with -> Archive Manager, then dig into it and find the file
@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.
@ilbert I see now that Hera is based on Ubuntu 18.04 LTS, but the file I listed above is for Ubuntu 16
So, try downloading the file from here - https://packages.ubuntu.com/bionic-updates/amd64/chromium-codecs-ffmpeg-extra/download and do the same thing.
Let me know how did it work.
@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
@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.
