As it seems, you are getting your chromium-related stuff from this ppa repo
which, as said in its description, is an unofficial repo for testing chromium related packages before they move to main repo. That's why you got chromium ffmpeg codecs v67 ahead of everyone else. Do you really need that repo? Do you get anything else from it, like chromium?
Don't recall when that ppa repo was added. It was quite awhile ago. Did notice that chromium browser upgrades more often than Opera. Running Chome Version 67.0.3396.87 (Official Build) (64-bit) right now.
Did your; sudo apt-get install chromium-codecs-ffmpeg-extra=66.0.3359.181-0ubuntu0.16.04.1 and got vidoes back on Opera 53.0.2907.99.
But did notice now on Opera 53.0.2907.99 when going to either a New Private Window, or using Opera VPN videos will not play on some sites, asking you to enable Flash! Turn off VPN or get out of New Private Window and videos play fine. Any idea why this happens?