This is a direct link to video:
Opera uses the Windows Media Foundation (as opposed to ffmpeg like Chrome) to play h.264 videos like this. For it to work in Opera, it needs to work in Windows Media Player first and be supported by WMF (a DirectShow plug-in on Windows won't do it). Then it should work.
If you have an "N" version of Windows 8, you might need https://support.microsoft.com/en-us/help/2929699/media-feature-pack-for-windows-8-1-n-and-windows-8-1-kn-editions-april.
However, I don't know Opera's status of support for Windows 8.1 and WMF. Maybe it only works for Win7 and Win10.
Test to see if the video works in Windows Media Player first.
You could also test in Vivaldi for another comparison. You just can't compare with Chrome or Chromium built with "all codecs".