What is Oracle equivalent for Linked Server and can you join with SQL Server?
Yes, you can.
In Oracle, this is called "Heterogeneous Connectivity" This Oracle admin manual has details. The basic outline is 


Install ODBC drivers on Oracle server
Configure ODBC to talk to the SQL Server DB (create a system DSN)
Test the ODBC
Configure Global_Names parameter to false.
Create initodbc.ora file to configure Heterogeneous services
Modify listener.ora to connect to both Oracle and ODBC drivers
Modify tnsNames.ora file
Reload the listener to see the above changes.
Create a DB Link from Oracle to the ODBC connection.
Test via SQL Select statement.


This article walks through an example case of doing the above steps with details.