Do you have an Opera in one or more of these places:
"C:\Program Files (x86)\Opera"
If so, just delete those folders. Then, try the uninstall from the control panel again. It should then ask you if you want to remove it from the list.
If you want to also delete Opera's settings and files, you can delete "C:\Users\yourusername\AppData\Roaming\Opera Software\Opera Stable" and "C:\Users\yourusername\AppData\Local\Opera Software\Opera Stable". Or, delete the parent "Opera Software" folders to get rid of all Opera profiles (if there are more besides "Opera Stable".
CCleaner has options to remove invalid entries from the registry. You could run that after doing the above if you want.
You could also use an uninstaller program if you want.