The solution depends on if you are using Oracle Internet Directory (OID) or not. If you are using OID you can login to the OID Console or the oidadmin GUI to reset the password easily.
But what if your application server is stand alone? If your trying to reset the oc4jadmin superuser account then the only solution is manually modify the $ORACLE_HOME/j2ee/home/config/system-jazn-data.xml file.
Search the file for your the username and place the new password, prefixed with an exclamation point "!" between the <credentials> </credentials> delimiters. I am assuming that you have to prefix it with the ! so that Oracle knows how to handle the password. ie, that its not encrypted.
Restart the application server and verify that you can login.
How about regular user accounts tho? Surprisingly you have to follow the same process. I must have poked around in the Application Server Console for half an hour in disbelief that I couldn't reset a user password without knowing the old password.
If I missed something or if you know of another way to reset user accounts, please post a comment.