Hi James,
It seems to me that you have nothing to do in the registry.
Once the OCS Agent is uninstalled on a Windows client, some installation files remain (such as the tag, plugins, etc.).
In order to complete the uninstallation, you must restart Windows and then manually remove the directories:
C:\Program Files (x86)\OCS Inventory Agent\
and
C:\ProgramData\OCS Inventory NG\
Regards, Stéphane