Try these steps to stop Opera from auto-starting:
Check Task Scheduler – Open Task Scheduler (Win + R, type taskschd.msc), look for any Opera-related tasks, and disable them.
Check Registry – Open Registry Editor (Win + R, type regedit), navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run, and delete any Opera entry.
Check Opera Settings – In Opera, go to Settings > On startup and make sure it’s not set to reopen previous sessions.
Reinstall Opera – If nothing works, uninstall Opera completely and reinstall it without enabling auto-start options.