There is no 10th or 12th board percentage weightage given for OBC or any category students for getting a seat in any of the IIT. Only thing is that you need to have atleast 75% in 12th board exam so that you can become eligible for JEE Mains exam and then clear the JEE Mains exam cut-off for your category which will make you eligible for JEE Advanced exam. If you clear the JEE Advanced exam cut-off in your category then you will be eligible to sit in the IITs seat allotment process but only clearing cut-off will never promise you a seat because 50,000 students are made eligible for counselling whereas there are only around 11,000 seats in all the IITs. So, you always need to aim for a high score so that you can get a good rank.

So, finally, the admission in IIT is entirely based on your performance in JEE Advanced exam.

Feel free to ask if you have any more queries. All the best!!