OSB provides extensive tooling for applications and servers that inspect, modify, and route message traffic. It layers on top of WL JMS, Web-Services, plus EJB and is intended to simplify the development and administration of relatively complex SOA topologies based on abstracting out common messaging patterns. (You may get a different answer from an OSB newsgroup.)
WL JMS provides the messaging "plumbing" for the Java EE DIY crowd. :-) You can use it for queueing & pub/sub of clients and server-side apps. You can also directly write your own Java EE MDBs to host server-side apps, directly use WebLogic Server's built in "Foreign JMS" mapping services to map a foreign JNDI JMS reference to local JNDI, plus use it's Messaging Bridges and/or SAF Agents services as a way to store-and-forward messages between machines.
For links to WebLogic Messaging white-papers, blog posts, samples, and documentation, click Enterprise Grid Messaging on the Oracle WebLogic Product Page.