How to uninstall oracle completely.

How to uninstall oracle completely. 

1.       Uninstall all Oracle components using the Oracle Universal Installer (OUI), click on remove button.

2.       If your OS is 32 bit then follow this Run regedit.exe and delete the HKEY_LOCAL_MACHINE/SOFTWARE/ORACLE key. This contains registry entries for all Oracle products.
3.       If your OS is 64 bit then follow this Run regedit.exe and delete the HKEY_LOCAL_MACHINE/SOFTWARE/ Wow6432Node/ORACLE key. This contains registry entries for all Oracle products.
4.       Delete any references to Oracle services left behind in the following part of the registry:
5.       HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/Ora*
6.       Reboot your machine.
7.       Delete the "C:\Oracle" directory, or whatever directory is your ORACLE_HOME.
8.       Delete the "C:\Program Files\Oracle" directory.
9.       Empty the contents of your "c:\temp" directory.
10.   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.