I currently have an open case with Esri support regarding this problem as well. At pro 2.1.3 I am able to easily connect to an Oracle database with a read-only user that has been created by our DBAs and is used to provide read-only access to the Oracle tables, including via our GIS tools. This user is also used in other departments by other applications that need this data.
Upgrading the ArcGIS Pro client to 2.2 broke this connection ability. Nothing had changed in the user credentials or any other settings in Oracle. While Esri support has been helpful, the most recent suggestion was to give this read-only user CREATE TABLE permissions in Oracle.
For those of us in larger organizations, and since we are using the Oracle tables to supplement our GIS data and create new datasets that we then make available as GIS data in our enterprise GDB (SQL server), we have no ability to alter permissions on the Oracle database--nor should we. We use SQL server as our storage RDBMS for our enterprise geodatabase--we can apply permissions here no problem. However, expecting GIS users to be able to alter permissions on citywide RDBMS system to make Pro able to connect just to see the data isn't realistic. In my opinion this should not be considered a viable workaround. I will follow up with whatever the final solution turns out to be--for now I have rolled back the version of Pro to 2.1.3.
Edit: (10/3/18) I was just informed by Esri tech support that this is a bug that will be fixed in next patch (2.2.3) for ArcGIS Pro.
