Recently Oracle gave access to download their new product Oracle ESB (formally known Aqua Logic ESB). Last year one of my project was related with SOA suite ESB. However, there were a few bugs with it, but we completed our project successfully with satisfaction.
When the times came to use Oracle OSB, i wondered with his functionality, a lot of Nodes and Actions pleased me well. At first it seems to be on workspace of BPEL but it's only for using on ESB. All these nodes made easy of ESB development. See the design template at a glance
However i have also found a cons in OSB is that, it haven't contains any built in JCA compatible adapter to connect with another resource. Throw proxy service OSB can use any type of protocol include FTP, SMTP, TUX. OSB defines these type of services SOAP without WSDL and XML without WSDL service. Take a look at the built in adapters in SOA suite ESB.
However, OSB hasn't contain any adapter, through proxy service OSB can integrate with any kind of legacy system and resources.
It would be better to take a glance in the following overall architecture of the OSB:
Link for download OSB.
A good book to learn OSB out here The Definitive Guide to SOA: Oracle® Service Bus, Second Edition.