If I'm not mistaken, SQL Developer allows you to export the tables via CREATE and INSERT statements. I believe there's also a tool in there that can move data from MSSQL to Oracle, but I don't know if it works the other way around.

A linked server is one option, but you might also want to consider creating something in SSIS. If you have the enterprise or developer version, there are special connectors you can use to connect to Oracle.

https://www.microsoft.com/en-gb/download/details.aspx?id=29284

Otherwise, you'll have to use OleDB or ODBC.