Getting the callback is the hard part. You have already managed to stand out from the many short OCI interviews as someone they would like to talk to more. The person who did your OCI is already on your side. They liked you enough to call you back, so you already have at least one vote in your favor. So just relax, and be yourself. You will probably get an offer from most of the firms that have called you back, unless you are a total tool. At this stage, the job is yours to lose.