Here is a sample startup script for my OC4J:
cd $HOME/oc4j/j2ee/home
java
-Djavax.xml.parsers.DocumentBuilderFactory=org.apache.xerces.jaxp.DocumentBuilderFactoryImpl
-Djavax.xml.parsers.SAXParserFactory=org.apache.xerces.jaxp.SAXParserFactoryImpl
-Djavax.xml.transform.TransformerFactory=org.apache.xalan.processor.TransformerFactoryImpl
-jar oc4j.jar
Xalan and Xerces libraries should be placed under JAVA_HOME/jre/lib/ext and remember to give at least read access to the account running the OC4j container.