I think that this is two problems
1. Link from MS SQL to Oracle.
2. Link from Oracle to MS SQL.

Link from MS SQL to Oracle is very simple question because you can create database link from MS SQL by ODBC. ( see Enterprise manager -> DTS )

You can create link from Oracle to MS SQL 2 way:
1. The best way Oracle Transparent Gateway.
2. Oracle Heteregenios Agent ( ODBC ).

You can see about this problem book:
ISBN: 0764546996