I've created an app using the free edition and connected to an oracle database in the modeller. However when I deploy it reverts to built in storage and I can't see anywhere to point to the same Oracle database as used when running locally. Is this possible?
No, you cannot point a Mendix free edition app to an Oracle database. It will always connect to the postgres database in the cloud. You can of course use Oracle with a paid license if you're hosting Mendix on your own hardware or cloud. In the Mendix cloud, you'll always be using postgres.