It depends...
You can use the External Database Connector to get data into NPEs, show them, make changes, and write data back to the Oracle RDBMS. You can also use the external database connector to copy oracle data into persistent entities, use those, and sync changes back to oracle.
What works best in your case depends on the type of pages and interaction you're looking for, the amount of data, the number of users, and the specifics of the oracle database.
I would aim for a solution where you don't have to make any changes to the Oracle RDBMS, so use the existing method of retrieving and updating data. Often Oracle applications will have stored procedures or packages for both retrieve and update of data. But sometimes this is coded in pl/sql in Oracle Forms.
If the database has a lot of plsql stored procedures or packages, i would aim to reuse these as much as possible as they will often have important business validations, and data initialization logic.
It may also be an option to hide the database behind an API layer, and build your UI app on top of these APIs. You can build the APIs with Mendix, also using the External Database Connector, or use a different product, e.g., Oracle ORD