Dear Akshata, it might just sound like inappropraite selection. But it does make lot of difference. It is a mistake in regards to selecting your Nationality. Now there are 2 things that might happen. Your fees structure and counselling procedure will change completely. And lets say you clear that hurdle, during document verification, because you have selected OCI by mistake and yoi show your Indian Nationality  document, unfortunately it will not be considered as a mistake but will be considered as submission of incorrect information and your admission might stand cancelled.