is 120 a good mark will I get above 90 percentile. I am obc

I hope you are asking about either JEE Mains Paper 1 or 2. Scoring 120 in either of the exams may not give the result with percentile of 90+. It is highly not possible to get 90+ percentile in the paper 2 with 120 marks and coming to paper 1, there are some rare chances of getting.

The percentile of a student is purely based on the performance in the exam. The allocation of seats by the students depends upon the category they belong to.

Gautham ;

120 marks out of 300 can be considered as good enough for you to qualify for JEE advance but can hardly get you into any top NITs. Being an OBC candidate you should go for 150+ to get a seat in good NITs.

Depending on the toughness of the paper the 90 percentile system will vary. Mostly 120 is enough for you to get above 90 percentile. But if you want to get into any top NITs you need to try a bit harder.

Hope this helps and all the best!