Hi all, whether you can see these videos or not depends on the graphics adapter used in your Mac (this is why some people can see the videos and others can't, using the same version of Opera and the same version of macOS).
On Macs that have graphics cards that can't be used for hardware-accelerated video decoding by Opera, Opera's stable version currently doesn't support playing back MSE/DASH videos ( https://en.wikipedia.org/wiki/DynamicAdaptiveStreamingoverHTTP ). These videos are used on Twitter, and in many cases on YouTube (especially for streaming videos).
The current developer edition of Opera (where we test experimental features and new stuff) includes a software video decoder for these cases. If you have this problem, please download Opera Developer from http://www.opera.com/blogs/desktop/ and let us know if this works for you. This feature will become available in Opera's stable version after testing and when we're sure it works as expected.
Wow. I was on my way to give up Opera due to those video problems - Opera Beta 41 seem to solve all my issues. yihaaaooo