Hi Divya!

If you are not able to see the OBC category option into the dropbox of reservation category, then the officials have not kept that as a reservation category. You can write a mail to the board regarding the same. They will guide you further on the details. In some cases OBC reservation is not considered. You can visit here for more information:

https://medicine.careers360.com/articles/indian-army-bsc-nursing-application-form

Thankyou!