Creating a "database link" from Oracle to SQL Server
The Oracle database server accepts SQL statements that query data stored in several different databases. The Oracle database server with the Heterogeneous Services component processes the SQL statement and passes the appropriate SQL directly to other Oracle databases and through gateways to non-Oracle databases. The Oracle database server then combines the results and returns them to the client.


Database Heterogeneous Connectivity Administrator's Guide

Oracle provides heterogeneous services to query non-Oracle databases like SQL Server. If you need some guidance here are some useful links.

 Connecting to Non-Oracle Data Sources Through Gateways

 How to Set up Third Party Databases Drivers in Oracle SQL Developer

 Heterogeneous Services - Generic Connectivity