Same issue and I have done the same validations. Can confirm that 2018.1 desktop works with 2018.1 Oracle drivers but Tableau Prep fails with the Oci error.
Similar issue. I can connect to Oracle from Desktop 2018.1 with the new 2018.1 Oracle Driver. I got the oci error with Prep 2018.1, but I hadn't downloaded the separate Prep 2018.1 Oracle Driver. I downloaded it and am now getting a new error (FYI, I'm inputting the same information into Prep to establish the connection as I do in Desktop):
An error occurred while communicating with Oracle.
Detailed Error Message
Oracle database error 12154: ORA-12154: TNS:could not resolve the connect identifier specified
Unable to connect to the server "Empower". Check that the server is running and that you have access privileges to the requested database.
Thanks Arthur - that fixed it for me. Tableau should reorganize that page a bit - all of the Tableau Desktop drivers are on top, and the one Prep driver is at the bottom. If you scroll a bit, all you see is old versions of the Oracle Tableau Desktop driver.