Upgrading JDK for existing 11g OBIEE is very simple as it is defined in Oracle Documentation here: https://docs.oracle.com/cd/E28280_01/bi.1111/e10541/patching.htm#BIESG9065
Generally if not said explicitly in Certification matrix it is assumed that any higher jdk6+ will be compatible with OBIEE 11g which is not in this case. That is weird !
But looks like its not that simple when you have followed the process and all services (Admin / Managed / Obis / Obips / opmn) up and running but heck no clue (not even from log) why the the user authentication fails.
The only clue can be given by weblogic Console when you launch it and go to Deployment components you will several weblogic components is in “Prepared” state rather “Active” . So this is your problem .
When you search for this error you would be able to know sun/io/CharacterEncoding is obsolete for JDK 8 and some of the dependent process will fail subsequently .
Now the only solution you would be having to resolve this problem to downgrade to JDK 7. Why I have faced this error is, I want to execute my bi-migration tool jar exported from JDK 8 (as in OBIEE 12c) and want to create a 11g export bundle for use in 12c as import archive. After making the JDK change I started all the services and want to see if my application working fine or not and found its broke.