How to delete the Opera browser from my Mac?

If an app was in fact forcibly downloaded and installed on that Mac, then by definition it is not exclusively yours to own and control. There is no point in providing deletion instructions. Even if they were successful, there is no limit to what else might be installed on that Mac without your consent.

To fix it, erase the Mac and reconfigure it as your own. Follow these instructions: What to do before you sell, give away, or trade in your Mac - Apple Support.

I do not understand how any app could be forcibly downloaded and installed. But in order to get rid of it you can try the following.

• Restart the computer in the Safe mode and try moving the browser app from the Applications folder to the Trash. Then empty it.
• Visit the Opera web site and download an uninstaller app if they have one there.
• Download and install the AppCleaner app. It will totally and safely uninstall all the bits and pieces of any app when you follow the directions.

Ronasara wrote:

I do not understand how any app could be forcibly downloaded and installed…

That can happen with supervised devices, and is one of the reasons why a device is supervised.

If this device is supervised, the user will have less or little or no control over its contents and management.

Including apps installed and locked against deletion.