I think you are asking whether you can connect from a client application outside the Oracle Cloud to a Database Cloud Service. The only way to accomplish this is through RESTful Web Services, which you have to define for each individual SQL statement and PL/SQL block.
If your client application uses this access method, it will work fine, but my guess is that it does not. So you can modify you client application to use this, you should be fine. If you want to simply use SQL*Net to point to your Database Cloud Service, this is not possible.
I think there is a misunderstanding here. The Oracle Database Cloud is primarily a PaaS service, while you are looking for a Database-as-a-Service offering. The difference is explained in this white paper, available at the cloud.oracle.com site - http://www.oracle.com/technetwork/database/database-cloud/public/oracle-db-and-db-cloud-service-wp-1844127.pdf.
Oracle does offer a DBaaS service, Oracle Managed Cloud Services (formerly Oracle OnDemand), which will probably deliver what you are looking for.