As of the OC4J 10.1.3 release, there is no longer a "default" or out-of-the-box administrator password.
The password to be used for the administation user (oc4jadmin) is set when OC4J is first started.
If you are using OC4J standalone, you will be prompted for the password the very first time you start the OC4J instance.
If you are using Oracle Application Server, you would have been prompted for the password during the installation process that would have then been set for the OC4J instance.
If you use the "Create Instance" button to create a new OC4J instance within Application Server Control, the password will be inherited from the initial OC4J instance.
The password for the instance is encrypted and stored in the system-jazn-data.xml file.