With the support of SCA within the Oracle SOA Suite 11g you get basically two busses: The Mediator (the "old" OESB) and (as a separate product) the OSB 11g. The Mediator is positioned as wiring intra Composite and the OSB to wire enterprise Services.
A question that puzzles me is if it is wise to (always) expose all Services via the OSB?
In that case the OSB is your abstraction Service entry layer and all Services are reachable and configurable through the bus. This way you use the loose coupling strategy of Proxy- and Business Services within the OSB.
What is your experience or opinion on this?