SSRS 2012 Report with Oracle database as datasource never finishes
I would recommend the following: 

1) Run the report and check Oracle's session (v$session) view to see if the Oracle database even registered the connection and the query was passed to it.

a) if YES, then check your report server's timeout settings. If the initial data return is elapsing the timeout period in either your report or your RS config file, this may likely be the reason for your timeout issue. You can check out report timeout periods in your data source configurations. You can check time out RS settings in your report server's config file.

b) if NO, you've got something else going that is more fundamental in connecting to the Oracle db and submitting the query.