You may already know more about this topic than I do, but in general, if you want Oracle to connect to a non-Oracle database, you need to install and configure the appropriate Transparent Gateway to the other DBMS. Then you can create a database link within your Oracle db that will allow you to make calls to the other db, as if it were really an Oracle db.
My install pack comes with a few Transparent Gateways, including SQL Server and Sybase, but not to Teradata. I suppose you are in the same situation, so the first thing you should do is contact Oracle to order the Transparent Gateway that you need.