Hi Chandra,
Oracle 12c was released later than ArcSDE 10.2.1, so I'm not sure if it will be fully compatible.
As I mentioned earlier, we haven't promoted using the "ArcSDE service model" in several releases so you don't need to install the ArcSDE software. To understand the difference between the "ArcSDE service model" and the "direct connect model", please see this older technical article,
1-Enterprise Geodatabase 101
(see the section titled Type of Client Connections)
This should help clarify the 2 connection models.
Hope this helps,