I'm not expert on this, so the following could be nonsense!
Doesn't a given booking class such as 'O' actually encompass several different fares within that class. E.g. one way TIA to LHR I can see
etc, all at different prices and rules. What I don't know is does O9 availability mean they will sell at least 9 of
any of those fares, or maybe just 5 of the cheapest, 9+ of the next and so on....
Maybe someone can clarify?