Are you talking about the portable version of Opera from https://www.opera.com/download (or the standalone installation in the installer's options) or are you talking about Opera from https://portableapps.com/apps/internet/opera_portable?
I'm talking about the portable version of Opera from https://www.opera.com/download
I changed [HKEY_LOCAL_MACHINE\SOFTWARE\RegisteredApplications]
I run tool again as admin: click unregistered
got to set default apps setting -> browser -> It Shows Up Now
done & confused how's it been working