I was having the same problem until I read this topic. Yes, the problem happens because of the flash player, not because of Opera. Chrome receives HTML5 version. Opera receives Flash version. I switched User-Agent in Chrome to Opera's. It received Flash version, and the video had the exact same issues you see in Opera. So I switched User-Agent in Opera to Chrome's. It received HTML5 version, and everything worked fine.

In other words, just switch Opera's User-Agent to Chrome. If you don't know how, the simplest way is to download an extension, like https://addons.opera.com/pt/extensions/details/user-agent-switcher/?display=en