Depends on when the flight is cancelled. If the flight is cancelled in advance (>24 hours of scheduled flight), in general they will not put you on OAL and instead offer a refund instead if there were no other UA options. However, you can always ask, but prepare to be disappointed.
For same day cancellations, there is more flexibility, but they will try to rebook on UA options first. If the rescheduled flights are not reasonable and an OAL option exists, the easiest way to convince the agent is to have those specific flight information available. The biggest challenge may be finding enough seats to accomodate the entire group, and this is where the group desk may be helpful.