You could do it with a database link, using Etherogeneous connectivity.
There are at least to ways to connect Oracle to non-Oracle databases (Transparent Gateways and Generic Connectivity).
Have a look at the following pages for detailed information:
Database link from Oracle to SQL Server (Oracle Tips by Burleson Consulting
Generic Heterogeneous Services and Transparent Gateways in 10 Minutes
Oracle® Database Heterogeneous Connectivity Administrator’s Guide
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