How to completely remove Oracle in Windows

In the past I've had many problems uninstalling all Oracle products from Windows systems. Here's my last resort method:

• Uninstall all Oracle components using the Oracle Universal Installer (OUI).
• Run regedit.exe and delete the HKEY_LOCAL_MACHINE/SOFTWARE/Oracle key. This contains registry entires for all Oracle products.
• If you are running 64-bit Windows, you should also delete the HKEY_LOCAL_MACHINE/SOFTWARE/Wow6432Node/Oracle key if it exists.
• Delete any references to Oracle services left behind in the following part of the registry (HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/Ora*). It should be pretty obvious which ones relate to Oracle.
• Reboot your machine.
• Delete the "C:\Oracle" directory, or whatever directory is your ORACLE_BASE.
• Delete the "C:\Program Files\Oracle" directory.
• If you are running 64-bit Wiindows, you should also delete the "C:\Program Files (x86)\Oracle" directory.
• Remove any Oracle-related subdirectories from the "C:\ProgramData\Microsoft\Windows\Start Menu\Programs\" directory.
• Empty the contents of your "C:\temp" directory.
• Empty your recycle bin.

At this point your machine will be as clean of Oracle components as it can be without a complete OS reinstall.

Remember, manually editing your registry can be very destructive and force an OS reinstall so only do it as a last resort.

If some DLLs can't be deleted, try renaming them, the after a reboot delete them.

(Source: https://oracle-base.com/articles/misc/manual-oracle-uninstall)