I experienced the same issue after the upgrade.
I installed the chromium codec through snap:

sudo snap install chromium-ffmpeg

linked the codec to opera lib-extra folder and restarted the browser.

sudo ln -s /snap/chromium-ffmpeg/37/chromium-ffmpeg-114023/chromium-ffmpeg/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/lib_extra/libffmpeg.so

NOTE: make sure to remove the old libffmpeg.so if exists.