OC4J expands to Oracle Containers For Java.

Originally based on the IronFlare Orion Application Server, has developed solely under Oracle's control since Oracle Corporation acquired the source.( this is the reason why you see config files are named as orion-web.xml, orion-application.xml etc.,)

In laymans terms OC4J can be described as "Oracle's Implementation of J2EE specification set".Sample specification set can be found here.For example you have finance application which got developed using JSP and servlet specifications.

You can package them as war/ear files and deploy them in OC4J containers which will run the applications as per the clients' request from web browser. There are lot more done by OC4J but above one was a simple example.

It will have xml based config files.

In R12 we have 3 groups of OC4J's. OC4J replaces Jserv(Java servlet containers) which came with earlier 11i techstack.
oacore OC4J - Supports framework based applications

forms OC4J - Supports forms based applications

oafm OC4J - expands to Oracle Application Fusion Middleware - for mapviewer, webservices, ascontrol

Number of OC4J instances for each group will be determined by corresponding nprocs context variable ( s_oacore_nprocs, s_forms_nprocs/s_frmsrv_nprocs, s_oafm_nprocs).-bash-

To be more precise, forms.ear application is deployed in forms OC4J to server forms based applications. ( $IAS_ORACLE-HOME/j2ee/forms/applications/forms.ear)oafm.ear, mapviewer.ear, ascontrol.ear applications (can be found at $IAS_ORACLE-HOME/j2ee/oafm/applications/) are deployed under oafm OC4J container.

But remember, even if one file change in ear file, will leads to creation of new ear file and redeployment, which is time consuming. hence the dummy ear file is used and config files are tweaked to support adpatching in ebiz R12.OC4J deployment will create specific directory structure and similar config files. I will cover them in detail in forthcoming posts.I hope You understood bit about OC4J