It is possible to run an application on premise and run your Oracle database on AWS on EC2 or RDS. However, in in this scenario, the network latency is typically to high and network bandwidth to limited to support most database systems. This is where AWS Direct Connect comes in. AWS Direct Connect provides speeds of 1 GBPS to 10GBPS where as a typical internet connection speed of 100MBPS (100Base-T connection) would be significantly slower. Keep in mind that an internal dedicated network from application server box to database server which would be 10 GBPS to 100 GBPS. If the application server and database server on on the same box there is no network latency. There are customers that run Oracle applications on EC2 and Oracle RAC databases in a co location facility using Direct Connect.