With Oracle SOA Suite 11g the old Oracle ESB become the Mediator component. With that only one "real" service bus resides, the Oracle Serivce Bus (OSB), which has been taken over from BEA (used to be Aqualogic Service Bus).
Mediator and OSB have some overlapping funcitonality, like transformation, routing and filtering. The question automatically raised is of course when to use which component. This presentation shows the difference between the components, the functionality they provide and some typical use cases for both.