How to connect to root with Oracle Developer VM
ALL of the passwords are 'oracle'

That includes:
 OS: root
 OS: oracle
 Database: SYS
 Database: SYSTEM
 Database: HR    

oracle@vbgeneric ~]$ sqlplus sys as sysdba

SQL*Plus: Release 12.2.0.1.0 Production on Fri May 11 11:43:25 2018

Copyright (c) 1982, 2016, Oracle.  All rights reserved.

Enter password: 

Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production

SQL&gt; 


However, you're running into something different.

And it's discussed here.


  Using TWO_TASK. A third way is by defining the TWO_TASK operating
  system variable to point to the PDB you want to connect to:


[oracle@prosrv1 schema]$ export TWO_TASK=PDB1


And, then you can connect as usual without giving a connect string:

[oracle@prosrv1 schema]$ sqlplus system/oracle


I prefer this last approach because it simulates a database connection prior to introduction of the PDB. If you connect to a specific PDB the majority of the time, all you have to do is to set this variable in the shell initialization file (e.g. .profile in case of bourne shell) of the user so that the variable is automatically set when you log in. If you need to know which PDB you are connected right now in SQL*Plus, just use the following command: To show which container (or, the PDB) you are connected to:

SQL&gt; show con_id
CON_ID
------------------------------
5

SQL&gt; show con_name
CON_NAME
------------------------------
PDB1