The only way to find out about O inventory is to call, because no site shows upgrade inventory for international flights. The only way way to get a feel about the upgrade situation is to see how many seats have been assigned in Envoy class and hope you get upgraded at the airport. The less assigned the better the chance to get an upgrade; however, there could be a number of seats that are purchased but have no seat assignment.

Here is a list provided by Chexfan:
A- Discounted First
B- Full Fare Economy
C- Discounted Business
D- First Class Award
E- Business Class Award
F- Full Fare First
G- Upgraded First from Discounted Coach (non-transatlantic)
H- Discounted Coach
I- Upgraded First from Full Fare
J- Full Fare Business
K- Discounted Coach
M- Discounted Coach
O- Upgrade to Business from Discounted Economy
Q- Discounted Coach
Y- Full Fare Economy
U- Shuttle Coach (unrestricted)
V- Discounted Coach
Z- Economy Class Award

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Code Summary