There's a lot of confusion towards the positioning of the Oracle Service Bus. In this post I'll try to clear up the issue as best as I can.
First of all, Oracle has already shown a convergence of BEA and Oracle FMW. This will continue even stronger in the upcoming releases, 11g in particular. The strategic platform as it will be introduced with 11g will consist of two main components: Mediator and Oracle Service Bus.
The mediator is an intra-composite mediation component within an application. It is responsible for brokering communications between components that make up a composite (conform Service Component Architecture - SCA). It will enable transformation, routing, event delivery and payload validation. The mediator is almost exclusively based on Oracle ESB (yes, the old Oracle Enterprise Service Bus).
The Oracle Service Bus (OSB) provides service bus capabilities for the entire company, again including standard functionality as transformation, routing, event delivery and payload validation. It's main function is to decouple intra-application communication from inter-application. Endpoint changes will not affect the internals of composite applications. The OSB is based on the Aqualogic Service Bus, augmented with key features from the Oracle ESB, especially JCA adapters, DVM, X-ref and JDev based design-time.
In the long run, I expect the distinction between these implementations to disappear, but I do like the current setup, as it differs between development of application and integration. I wouldn't be surprised to see this pattern appear more.