How to create and start a PDB for RCU in Oracle 12c
Look at:
http://docs.oracle.com/database/121/DBSEG/authentication.htm#DBSEG99827

Operating system authentication is not supported in a multitenant environment.


You can't connect with "sqlplus / as sysdba" to a PDB. It only works with the CDB.

If you change environment variable ORACLE_SID to that of a PDB, you'll connect to the binaries to what he expects to be a CDB that doesn't exist, and you'll get the message "connected to an idle instance"

Fun fact: If you are logged as the oracle binaries owner (or someone in the dba group) and try to connect with: 

sqlplus pdbuser/pdbpass@pdb as sysdba


Even if the pdbuser doesn't exist in the CDB, you'll log in it, as pdbuser, with SYSDBA permissions, on the CDB, because of OS Authentication.

Or at least that's what happens in my test/experiment environment...