To fix the Twitch buffering issue on Opera, you should:

Check your network condition: make sure it is well-connected and strong enough. Change your DNA to Google’s DNS if possible. Update your browser to the newest version, or simply uninstall and reinstall your Opera. Clear cache and cookies of your browser. Disable the extensions or addons on your Opera. Turn off Hardware Acceleration of Opera. Switch to a more powerful VPN if you are using one to surf the Internet.

If you have done the above jobs to your Opera browser, but nothing changes, you can also try the following methods found online:

Change your browser: there are many browsers you can choose from including Chrome, Firefox, IE, etc. Close the unused background applications on your computer. Upgrade your computer's hardware for HD/4K streaming.

Resource: https://bit.ly/3xuLgNT