11gR2 didn't ask me to set username or any password
If the Universal Installer included with 11gR2 did not ask for a password, it is likely that the option to create a database was not selected during installation. 

Solution? Create a database with the dbca tool and supply a sys password when prompted. 

With older versions, perhaps including Oracle Database XE (11g), you might have luck with one of the default password combinations of sys/change_on_install or system/welcome1 or system/manager. On engineered systems I have also seen sys/welcome1 and sys/changeme