A number of ways spring to mind, but as for the most efficient that depends on your hardware, network and setup.
Solutions would be replication (Oracle can be a publisher), standard SSIS Oracle Source SQL Destination data flow tasks, BCP Out from Oracle, BCP In to SQL or SSIS In to SQL, linked servers.
Again the list goes on so it would be a case of pick a couple of ways and see what works best for your environment.