Windows
Oracle uninstall is a Dangerous Process. If you mistake any process then a lot of problem arise. I am searching a lot of and test it in oracle 10g. It works properly.

1.Uninstall all Oracle components using the Oracle Universal Installer (OUI).
2.Run regedit.exe and delete the HKEY_LOCAL_MACHINE/SOFTWARE/ORACLE key. This contains registry entires for all Oracle products.
3.Delete any references to Oracle services left behind in the following part of the registry:
HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/Ora*
4.It should be pretty obvious which ones relate to Oracle.
5.Reboot your machine.
6.Delete the “C:\Oracle” directory, or whatever directory is your ORACLE_BASE.
7. Delete the “C:\Program Files\Oracle” directory.
8.Empty the contents of your “c:\temp” directory.
9.Empty your recycle bin.

Source : http://www.oracle-base.com/
Link: http://www.oracle-base.com/articles/misc/ManualOracleUninstall.php