To set the operating system user for a PDB:
1. Log in to the root as a user and execute the DBMS_CREDENTIAL.CREATE_CREDENTIAL procedure to create an Oracle credential for the $user operating system user:

begin
		 DBMS_CREDENTIAL.CREATE_CREDENTIAL (credential_name => 'CDB1_PDBSW_BUSH'
              , username =>'bush'
              , password => 'passwd');
end;
/

PL/SQL procedure successfully completed.

2. Connect to the PDB to set the PDB_OS_CREDENTIAL to the new credential:

11:18:46 (1)[CDB$ROOT]sys@orcl> alter session set container=SW;
11:19:00 (1)[SW]sys@orcl>  ALTER SYSTEM SET PDB_OS_CREDENTIAL=CDB1_PDBSW_BUSH SCOPE=spfile;

3. Restart the CDB instance.
4. Test the connection as bush:

     $ sqlplus sys@pdb1 as sysdba 

not working =(
ps:
PDB_OS_CREDENTIAL
Undocumented
Per feedback by the Multitenant team:
Not functional in Oracle 12.1.0.2
May be functional with a future PSU allwoing then OS user verfication/validation for PDBs