Julian's suggestion could be tried.
However, OCP2b have mentioned that Oracle selects the RBS randomly. Try to allocate RBS of appropriate size depending upon the requirement, like:
1. Setting the parameter ROLLBACK_SEGMENTS=(rbs1,rbs2,....). Oracle will allocate rbs1 for the first assignment and rbs2 for the next ..........Hence arrange the RBS as per requirement.
2. Assigning rollback segments explicitly for the transactions.