Here is an article I wrote on Hetergeneous Services.
In order to connect to SQL*Server from Oracle, you
must do several things:
1. Install ODBC drivers for the SQL*Server database.
The drivers are installed on your laptop or on
the server that contains the Oracle code tree.
2. Setup the ODBC connection on your laptop using
the Windows ODBC Data Source Administrator
3. Test the ODBC drivers to ensure that connectivity
is made to the non-Oracle database.
4. Ensure the Global_names parameter is set to False
in the Oracle database.
5. Configure the Hetergeneous services. This
consists of creating an initodbc.ora file within the
Oracle database installation.
7. Modify the Listener.ora file on the database
installation to connect to the Oracle instance and
8. Modify the Tnsnames.ora file to point to the
proper code tree.
9. Reload the listener
10. Create a database link on the Oracle
11. Run a Select statement for the Oracle
installation using the database link.