what is the admission cutoff for obc category
It's not much. If you need around 99.5%+ as a General Candidate to get calls from a particular IIM, you will need around 97/98+ as an OBC. Similarly for grades- you need if you would need around 8.0+ to be safe as a general category, you will need 7.0+ as an OBC.
You cannot take the exam lightly, although it is relaxed for OBCs, in absolute terms it is still quite easy to miss the call. Of course- it ranges- if you are a woman with decent board marks as well as an OBC you might get calls from IIM-K/L/I at <90%ile as well.