Hi Ferox,
The OutSystems Entities are created in the database that is selected during Platform installation. The are names OSSYS_xxx for system tables (where xxx is the Entity name, including the User Entity), and OSUSR_xxx_yyy for "user" tables (i.e. Entities you defined yourself, where xxx is a three-character prefix that's derived from the eSpace name, and yyy is the Entity name*).
There's a system Entity, called Entity (what's in a name) and reflected in the database as ossys_Entity that maps the Entity name and the physical table name. You can reference the Entity from the Manage Dependencies pop-up, Producer "(System)":
If you have external tools that need to access a certain table, you can use ossys_Entity to find out what the physical table name is, and use it accordingly.
*If you have a very old installation, it may be an abbreviation of the Entity, since older versions of MSSQL (and possibly Oracle) didn't allow for long names.