I've just installed ODPwithODAC 12c and I was able to connect to Oracle 12c Database successfully, all the tables was loaded in the designer but without any relations (foreign key constraints) between them.
I am using Windows 8.1 (64 bit) ,Visual Studio 2013 Update 4, latest EF (6.1.2), ODPwithODAC 12 c Release 3 (32 bit) , and the Oracle Database is 12c (PDB) .
am I missing some configuration to get the the relations with the tables ?
Note: I've tried the same thing with Oracle 11g and I get the tables with their relations without any problem, also I tried Oracle 12c DB with EF 5 and once again it works great without any problems.
This sounds like a known bug that is specific to Oracle Multitenant scenarios. You should use database version 12.1.0.2 or later and make sure you have installed Oracle Developer Tools for Visual Studio version 12.1.0.2 or later.