There have been questions in regards to OC4J vs a "full" Application Server when in use with Oracle Business Intelligence Enterprise Edition.
To be clear OC4J is very capable J2EE engine. It is used as the J2EE runtime component of Oracle Application Server. However, when it comes to enterprise deployments the System Management, features, and functionality of the "full" Application Servers provide some benefits that the following document discusses.
OTN:
Enterprise Deployment of Oracle BI EE on OC4J and App Servers
My Oracle Support:
Note: 968223.1 - Enterprise Deployment of Oracle BI EE on OC4J and App Servers