I've never had a problem as long as I book at 180 days. Going at such a busy time, it would be the first ADR I would attempt, unless you have a BOG dinner also, in which case I would do that first.
We are going 9/22-10/1. We got Ohana the first night of our trip and the last night booked at 180 days. We have since dropped the first night reservation.
While searching for a new reservation for ohana between 12/8 - 12/10 would it be better to search for 2 or 4 people for better chance of finding a table and then adjusting the reservation if necessary to 2 people?