You will need to use the Oracle Open Client Adapter for ODBC (OCA) to achieve this.
A file called “oca_info.pdf” should be included in your Developer installation, which is usually at “ORACLE_HOME\TOOLS\DOCXX\US”. In this file you will find all the information you need to connect Developer to other databases (including SQL Server).
If you don’t have that file, you can download from the web <a href=”http://www.4shared.com/file/55333870/d9f1c359/oca_info.html”>here</a>.
You can find other useful article <a href=”http://searchoracle.techtarget.com/expert/KnowledgebaseAnswer/0,289625,sid41_cid560685,00.html”>here</a>
Hope this helps.