We don't do a lot of "O" tours, however a couple of times a tour was sold out. We put are name on the waitlist. Didn't think about until we got on the ship, we clear one right after boarding the ship. The 2nd one clear a few days later, they notified us and ask if we still wanted it.
Also when we booked tours on any ship, once we decided we didn't want to tour as much, we turned those back early on, so someone on a waitlist got them. Now touring isn't much for us, so we book very few for any cruise.