Yes! The Reports Module does support external databases.
You need to configure your Reports Module to use the "Generic ADO.Net" Data Source (in the Module Settings page) and then enter your Oracle Connection String and a Query. You'll need to select the OracleClient ADO.Net data provider (I believe it's installed by default, if not you'll need to install the Oracle ADO.Net Provider on the server and it should automatically appear in this list). You should be able to use any Visualizer you want, or even write your own.
Also, since it is open-source, the Reports Module makes a good starting point for a custom data-oriented module if the Reports Module itself doesn't quite support all your needs.
FYI: If you wish to use the "UserId", "TabId", "PortalId" and "ModuleId" parameters (which are passed to your query automatically and contain the User, Tab, Portal or Module ID, respectively), you must make sure to set the Parameter Prefix appropriately. For example, in Microsoft SQL Server, all parameter names must start with an "@", so the Parameter Prefix would be "@". I don't know if Oracle has such a restriction and what it is, so I can't advise you directly on that part.