I had some time to play around. On Debian (amd64) based systems (Debian, Ubuntu, Mint, ect) you need to uninstall
chromium-codecs-ffmpeg-extra with the command:
sudo apt-get remove chromium-codecs-ffmpeg-extra

You can uses this version as replacement:
https://launchpad.net/ubuntu/xenial/amd64/chromium-codecs-ffmpeg-extra/64.0.3282.119-0ubuntu0.16.04.1

or as direct download:
http://launchpadlibrarian.net/354818683/chromium-codecs-ffmpeg-extra_64.0.3282.119-0ubuntu0.16.04.1_amd64.deb

download the file and double click on the file to install it. Afterwards completely restart Opera.

If you want to prevent further updates of this package use this command (not recommended):
sudo apt-mark hold chromium-codecs-ffmpeg-extra

@dilanmanojI just updated the extension to support dark mode. Please note that the new version needs to be validated by Opera, which can take several months. Once approved, the extension will automatically update. If you don't want to wait that long, you can download the extension from the Chrome Web Store. You'll need version 1.6.0. Typically, the Chrome team validates extensions faster.

@minho I currently stay with version 66. This version is usable. I don't have time to beta test every new Opera Version.

With the newest Opera version the freezes are gone in the VM. Sadly is this version constantly freezing/crashing on my Thinkpad with Linux Mint 19.3. I don't know if the Devs at Opera are testing their Linux build proper. But this is really annoying. In the VM I will use the latest Version and on my Thinkpad I will stay with 66.0.3515.72 and the ffmpeg fix.

Copying the *.so file also helps if you have problems with video-playback with Opera 66 based on Chrome/79.0.3945.130. I tried this because I could not find a matching ffmpeg version so far.

@robomike

I tried to disable hardware acceleration. I did this in VirtualBox and also tried the Opera setting. Did not fix my problem.
I think the problem is related to this sandbox issue...

I have the same problem under VirtualBox and Mint 19 as Guest. Opera is not usable anymore. It constantly freezes. On my native devices the bug does not appear.

I just can agree. The old behavior was much better. Opera once was known for letting the user decide. If they really want to optimize the performance for users then this is ok for me. But they should just add a option to choose wich version you want. On my 8 core system with 24 GB I just don't care if it ist more performant the new way. It is just more inconvenient !!!

I had some time to play around. On Debian (amd64) based systems (Debian, Ubuntu, Mint, ect) you need to uninstall
chromium-codecs-ffmpeg-extra with the command:
sudo apt-get remove chromium-codecs-ffmpeg-extra

You can uses this version as replacement:
https://launchpad.net/ubuntu/xenial/amd64/chromium-codecs-ffmpeg-extra/64.0.3282.119-0ubuntu0.16.04.1

or as direct download:
http://launchpadlibrarian.net/354818683/chromium-codecs-ffmpeg-extra_64.0.3282.119-0ubuntu0.16.04.1_amd64.deb

download the file and double click on the file to install it. Afterwards completely restart Opera.

If you want to prevent further updates of this package use this command (not recommended):
sudo apt-mark hold chromium-codecs-ffmpeg-extra

H.264/MP4 problem in Opera 49.0.2725.5

Ok, I had some time to play around. On Debian (amd64) based systems (Debian, Ubuntu, Mint, ect) you need to uninstall
chromium-codecs-ffmpeg-extra with the command:
sudo apt-get remove chromium-codecs-ffmpeg-extra

You can uses this older version as replacement:
https://launchpad.net/ubuntu/xenial/amd64/chromium-codecs-ffmpeg-extra/61.0.3163.100-0ubuntu0.16.04.1306

or as direct download:
http://launchpadlibrarian.net/338867652/chromium-codecs-ffmpeg-extra_61.0.3163.100-0ubuntu0.16.04.1306_amd64.deb

download the file and use:
sudo dpkg --install chromium-codecs-ffmpeg-extra_61.0.3163.100-0ubuntu0.16.04.1306_amd64.deb
to install it, or double click on the file. Afterwards completely restart Opera.

If you want to prevent further updates of this package use this command (not recommended):
sudo apt-mark hold chromium-codecs-ffmpeg-extra