Sorry, should have explained! Thanks to the poster above who did.

Y is full-fare economy. O (and other flare classes) are deeply discounted. Generally, say there are 100 seats available. All of them should be available to purchase at the Y price, maybe 50 of those available for a slightly discounted price in H inventory, and a smaller subset of those, say 10, at the deeply discounted O inventory.

ExpertFlyer shows a maximum of 7 seats available, indicating that here are at least 7 seats available for sale in that inventory. There could be 7 available. There could be 64. If it's O6 on EF, it means 6 and no more.

The fact that it is O7 means that there are 7 or more seats available from that deeply discounted inventory, indicating that the flight is PROBABLY pretty open. Not necessarily empty or that there would be 64 empty seats, though.