Hey nidhi,

For filling DU form you need a OBC NCL (non creamy layer) certificate which is signed by the central government. The state OBC certificate isn't required. You need to get a central OBC certificate in order to take admission in DU. OBC-NCL is a certificate issued by central govt to people who are under central obc list and whose income is under 6 lakhs p.a. 

Hope this helps!