OC4J is Oracle's new J2ee container.
According to the benchmarks released by oracle and very according
to BEA's response to those (you need to read them to find out what
I'm talking about) oc4j  - which is based on Evermind's Orion - is
the fastest application server  currently available.
Please do not confuse oc4j with the previous versions of oracle's
application server.
If you are interested for more official documents and more specification
about this very interesting application server, I can send them to the
list.