From the way I understand it, the libffmpeg.so file that Opera uses doesn't contain the proper codecs for certain videos. This is how I fixed it for KDE Neon (Ubuntu variant). Make sure you have the Chromium browser because this will break Opera in the process and you will need the Chromium libffmpeg.so file. Use this at your own risk.

sudo updatebd (update locate database if you never used it before)

sudo locate libffmpeg.so (shows location of all libffmpeg.so files)

sudo rm /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so (removes the libffmpeg.so that Opera is using, if your results are different from the locate command then use that path)

sudo cp /usr/lib/chromium-browser/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/ (copy the Chromium libffmpeg.so into the Opera directory)

Opera ver 39.0

As I said, the syncing feature doesn't work properly. The only way to sync bookmarks is to copy and paste them into the "Unsorted Bookmarks" folder. The syncing feature does not sync the rest of the directory. When pasting bookmarks into the Unsorted Bookmarks folder, you can't create new file structures so everything is as you paste and move it. There is no "new folder" structure in Unsorted Bookmarks. I would still rather have my pre-created file system even though it might have been imported from another browser. Yes, importing from another browser creates an "Imported Bookmarks" folder and that is where your bookmarks are stored complete with the previous file structure.

Thank you for your time,

Jaime