Very often, in order to ensure the OS is "clean" before installing the Oracle products,
I usually would use OUI to remove the Oracle products left on that machine and then perform a manual removal of Oracle components.
Below is the steps I usually use to remove Oracle components after uninstalling with OUI.

1. Go to the Contrl Panel, stop all Oracle related services

2. Run regedit32, go to the following keys and delete them;

HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE
**Note the value of ORACLE_HOME and inst_loc(inst_loc is the location of the OUI), we need to these files later.

HKEY_LOCAL_MACHINE\SOFTWARE\ODBC and delete all subkey related to "Oracle ODBC Driver"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services and remove all keys that begin with Oracle ir ORAWEB

3. Delete the Oracle information from Environment Variable PATH

4. Go to %SystemDrive%\DOCUMENTS AND SETTINGS\ALL USERS\...\START MENU\PROGRAMS
Delete all Oracle icons

5. Delete the files in the directories specified in ORACLE_HOME and inst_loc keys

6. Restart Computer