If you have access to the Tableau Server computer and can run Tableau Desktop on it that will help debug this.
If your Server is inside your firewall, it may be missing the TNS file that contains the connection settings for Oracle. Or your Oracle database may be configured to only accept connections from certain IP addresses.
If your Server is outside your firewall it will not be able to connect to your Oracle database, you'll have to publish a pass-through data source from Tableau Desktop to Tableau Server that is supported by using Tableau Bridge.
I can help more once we figure out your setup.