Hey Abhishek!

I don't clearly get what you mean by failing in JEE ADVANCE exam because if you will fail , then you will simply not  be  considered neither as general nor as OBC candidate. But if the case is that your score is more than OBC candidate cutoffs but less than general candidate cutoffs ,then  you will obviously be considered as a General candidate because you have not uploaded your OBC certificate so they have no proof of you being an OBC candidate. So, if you want to be considered as an OBC candidate  you have to upload your OBC certificate at the time of the registration.

It is a certificate only that gives them a valid proof of being an OBC reserve candidate . So if there is no certificate means there is no valid proof of you being an OBC candidate, so they will consider you as a General candidate

For knowing more about JEE ADVANCE counselling, follow the given link ;

https://engineering.careers360.com/articles/jee-advanced