I cannot play most of the videos on Opera as well, too. (Youtube is working though.) For example, I tried videos on Twitter and bbc.com.
I can watch the videos on Firefox, Chrome, Vivaldi all right.
At https://html5test.com I can see that the difference between Opera and other browsers is the "No" for H.264 support and AAC support. (There is "No" for H.265 support and MPEG-4 ASP support as well, but none of my browsers have that anyway, so that's probably not the problem.)
I have disabled "Use hardware acceleration when available" and enabled "Override software rendering list", and I cannot find the flag "disable-accelerated-video-decode".