Its not mandatory to have Apps installed.
And more over, you dont need to install Oracle or any Back end too.
For learning purpose, OBIEE has default XML files data loaded, so that you can play around.
Or else you can create data in excel too and have them imported.
I have installed oracle database EE 11gr2 and after creating repository, installed OBIEE 11g successfully. After that I have created sample application and uploaded the .rpd file in weblogic server also. but when i am trying to restart the BI server components, I am getting error saying ' All 5 components are down'. any obiee experts or pointer will be of great help.