ODAC 12c and Entity Framework 6
Devart recently announced Oracle 12c support. You may want to consider using their provider instead.
**Oracle Data Access Components 12c Release 3 Beta 2 **ODAC 12c R3 is the first ODP.NET release to certify with Entity Framework (EF) 6 and EF Code First.http://www.oracle.com/technetwork/topics/dotnet/whatsnew/index.html
The error you're receiving states that you didn't add a Oracle.ManagedDataAccess.Client to the providers tag.
Adding this will solve your problem:
<provider invariantName="Oracle.ManagedDataAccessClient" type="Oracle.ManagedDataAccess.Client.OracleClientFactory, Oracle.ManagedDataAccess, Version=4.121.1.0, Culture=neutral, PublicKeyToken=89b483f429c47342" />but unfortunately will not work in the end due to lack of support for EF6 in ODAC 12c R2, as mentioned in the comments.