Dear Pravin,  JEE Mains must have asked you for your category details during the form filling. You will be asked for a scanned copy of your OBC-NCL during form filling for JEE Advance. This certificate should have been taken in the last 6 months. JEE Advance also gives you a time frame of 1 month to submit your certificate. Only when you submit it, will your OBC rank of JEE Mains will become activated.  Now if you don't appear for JEE Advance then the OBC NCL cerificate will be asked during JOSSA counselling session during reporting.