Pre 11G, in OIM, the password change for OIM administrator user (xelsysadm) can be reset by another admin via the Web console. Alternatively change the password using SQL in the OIM USR table where usr_login = XELSYSADM. There are a number of blogs which provide details of how to update the password in the USR table.
If 11G is using workflows, there is another location where the password needs to be changed.
Using Enterprise Manager (eg http://my.domain.name:7001/em), under farm_base_domain, click WebLogic Domain, click your base_domain.
This will open up the base_domain in the main window. On top left hand corner drop down WebLogic Domain, select Security, then select Credentials.
This will open up the Credential Store Provider window. Open the Credential called "oracle.oim.sysadminMap". Select the entry called Sysadmin. Use the Edit option to open the editor window. This should pop-up Edit Key. This should have a user name of the system admin (xelsysadm). Change the password here.