What rank do I have to attain to secure a seat in IIT for B.Des? I am from OBC category.

If you have to gain a seat in IIT, with top listed branches, your rank should be under 4000, as it will provide the maximum opportunity to get good branch of your choice.

10,000 general. For decent branch in decent IIT. One of my friend got 11k rank and he got admission in IITK. His branch was earth Science. Then he changed his branch to civil. It is easy not difficult to change branch.