First thing to do is to stop OC4J. For Unix users they can do it with the follozing command: ORACLE_HOME/opmn/bin/opmnctl stopproc ias-component=OC4J but for Windows is much easier:

Next thing to do is to locate and open the file "system-jazn_data.xml":

Find into this file the following tags and replace the text for
- must contain at least five characters, but not more than 30
- must begin with an alphabetic character. It cannot begin with a number, the underscore (_), the dollar sign ($), or the number sign (#)
- at least one of the characters must be a number
- can contain only the following characters: numbers, letters, and the following special characters: US dollar sign ($), number sign (#), or underscore (_)
- cannot contain any Oracle reserved words, such as VARCHAR.

After replacing the old encrypted password with a new one (don't worry about the encryption because after restarting the OC4J it will encrypt the text you entered there all by it's self) we just have to delete cached password data from C:\oracle\bise1\bi\oc4j_bi\j2ee\home\persistence\ascontrol\ascontrol\securestore

In the end you just have to start OC4J and try logging to the OAS with the new one.
Good luck and have fun!