There are 2 products you can use to link SQl Server databases into Oracle.
The first product is for free and it is called Database Gateway for ODBC which uses a 3rd party ODBC driver which is able to connect to a MS SQl Server database - the second one is a more powerful gateway which is designed for SQl Server connections and it is called Database Gateway for MS SQL Server.
DG4ODBC is based on a 3rd party ODBC driver and it allows you to connect with a suitable ODBC driver to any foreign database. So its capability is very limited. As mentioned, the second product is DG4MSQl and it is designed for MS SQl Server connections. It allows you also to call remote SQL Server stored procedures and also to participate in distributed transactions.
Bot gateways allow you to define in your Oracle database a database link which you can then use to collect the SQL Server data.