I had same issue. I changed just one flag:
opera://flags/#use-angle - from value "Default" to value "OpenGL".
And it helped me, now h.264 video playing correctly!
Test video: http://demo.nimius.net/video_test/
Opera v81.0.4196.60 stable.
Controversial posts made by clafix
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RE: The Videos Don't Play TopicOpera for Linux