There are a few possible reasons why your WordPress and WooCommerce admin panel is not displaying properly in the Opera browser. One possibility is that there is a compatibility issue between Opera and the WordPress or WooCommerce plugins. Another possibility is that there is a conflict between one of the plugins and the theme. Finally, it is also possible that there are corrupted WordPress files.

To troubleshoot your problem you can try my following recommended solutions.

1- Deactivating all plugins and then reactivating them one at a time to see if any of them are causing the problem.
2- Switching to a default WordPress theme to see if the problem is caused by the current theme.
3- Clearing the Opera browser's cache and cookies.
4- Updating WordPress and all plugins to the latest versions.

I tried this solution for my personal WooCommerce website and issue got resolve. If you have tried all of these steps and the problem persists, you may need to contact your web host for further assistance.