Oracle 12c Client to 10g Server (OPENQUERY)
As Oracle support note "Client / Server Interoperability Support Matrix for Different Oracle Versions" ( Doc ID 207303.1 ),  Oracle Client 12.1 does not support connections with Oracle DB servers 10.1 and below. 

Connections with Oracle DB 10.2 are still supported, but not recommended.

The recommended option its to use Oracle 10.2 client.