To make a Linked server to Oracle you need to do the following steps
1. Install an Oracle Client on the server on which you are trying to make a linked server.
2. Add a linked server using Microsoft Ole Db Provider for Oracle.
Productname : Anything
Data Source : SQL Net Name for Oracle
Provider : Anything
3. In the security tab in "Add new linked server " in Enterprise manager , add a linked server login and select the "Be made using this security context : " option . Enter the remote login and password here too.
4. Select options you need in the Server Options Tab