Is it possible to have something like Union-FS for Oracle databases?
You might be able to construct something like this using database links and views, but performance of those views in the development environment would likely be horrendous. That said, in general it is considered an extremely poor security practice to have production data be visible to non-production application instances, or to share any infrastructure (servers, network, storage) with them. In most computing environments it is specifically prohibited by best practice, if not by law, depending on the type of data being stored. 

In most of the places I have worked, according to the rules developers were not even allowed read-access to production data for any reason; no links between dev and production were allowed; and no copies of production data could be placed in dev environments. Any exceptions required senior management and security approval from multiple organizations and generally meant that dev environments wound up secured to the same level as production.