If you are running an Oracle database on a UNIX or Linux platform and want to read/write data in a remote SQL Server database, you can use Oracle's Heterogeneous Services (DG4ODBC/hsodbc) and the SQL Server ODBC Driver.
Oracle's Heterogeneous Services allows you to use Oracle SQL to transparently access data stored in non-Oracle systems as if the data was stored on an Oracle server.
You can find detailed instructions on how to set this up in the following articles:
Download the ODBC driver for SQL Server
You can also use the ODBC-ODBC Bridge to connect to any remote Windows database from Oracle.
You can find other ODBC Drivers that you can use with Oracle's Heterogeneous Services here.