It's not a linear progression as far as I understand. So it doesn't sell 9 O class and then zero it, and then 9 Q class etc.

The additional bit here is buying them in one booking. So had you done it in two and say bought 5 first, it wouldn't necessarily mean the flight became O4. You second booking for the remaining 4 may not have priced up in O as your first booking could have caused O to go below 4.

There is also point of sale availability, are you checking availability for the correct point of sale? Basically if the POS you are actually using for the booking is O5 and you look for 6 or more it will price them all in the next one up so Q. It wouldn't price it in one booking as 5 in O and 4 in Q.