@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.
Thanks, this works for me, Opera 75 on Debian 10.
Lol, I even reported a bug to Opera team before this.