This is because YouTube mainly serves VP9/webm videos to Opera (Google's own format, preferred by them). Safari doesn't support this format, so it gets H264 videos (which generally can be decoded with hardware acceleration, unlike VP9).
There's a Chrome extension called h264ify that changes YouTube to serve H264. You need to install this first in Opera:
https://addons.opera.com/en/extensions/details/download-chrome-extension-9/?display=en
(read the instructions)
Then you can install h264ify from here: https://chrome.google.com/webstore/detail/h264ify/aleakchihdccplidncghkekgioiakgal