Goto the URL opera://flags/#disable-accelerated-video-decode, disable the flag, and restart Opera.
If that doesn't help, revert the flag to default. Then, try different settings for the opera://flags/#use-angle flag (restarting Opera each time you change it).
If that doesn't help, revert the flag to default. Then, goto the URL opera://settings/system and disable hardware acceleration (restart after the change).
If that doesn't help, turn hardware acceleration back on and restart Opera. Then, goto https://html5test.com/ and see what it says about h.264 and aac support?
Also, is your edition of Windows 10 an "N" edition?
