Oracle Service Bus (previously known as Weblogic ALSB) is a light weight messaging component of SOA Suite. In addition to providing basic routing and transformation services, OSB also provides advanced capabilities for service abstraction.
As of version 18.104.22.168, OSB can only run on Weblogic platform. Since it runs in a separate JVM, there is an overhead in communicating with rest of SOA Suite components.
Though OSB is similar to Mediator component at the conceptual level, there are specific scenarios when OSB will be very useful.