10th: 9.8cgpa, 12th: 82%, BSc:75%, What %ile do I need to enter old IIMs with OBC category ?

With OBC category and decent past academic records you have chances to get in old IIM's but you will have to be good enough with the CAT percentile, and later selection rounds like GD/PI and WAT.

For old IIM's you will need percentile above 98 and for new IIM's target above 93 percentile. Though the difference between general and other caste category cutoffs are not much different.

You can check the selection process and admissions for first 3 IIM's below:

IIM B

IIM C

IIM A

Good Luck!!