Each class has different fares within it. The prices of these fares vary according to whether the ticket is refundable, flexible, upgradeable and/or requires advance purchase.

O is one of the cheapest Economy fares because it is non-refundable, non-flexible and non-upgradeable. Due to the low cost of the ticket, it doesn't earn the same number of miles as the more expensive Economy fares.

However, since you're saving £00s compared to the higher fares, the additional miles aren't worth the extra cost.

In terms of the actual ground and onboard product, there is no difference between the expensive and cheaper fares.

Hope that helps a bit.