Both of my kids have had extensive OT, so I have just a few thoughts.
I have seen the OT use 6" diameter rings that the child places over a cone, but that was only part of the entire exercise. DS would be lying flat on his stomach on the "pizza swing"- a triangular board suspended from the ceiling that swings in all directions. The rings would be placed on the mat below the swing and the cones placed to one side. The therapist would pull the swing toward her and let go, DS would swing back and on the way forward again he was to pick up a ring, swing back again, and then on his way forward put the ring on the cone. This exercise took a loooooong time for him to master and they worked on it a little at at time. At first he couldn't even stay on the swing! Then he had to learn to adjust his body weight with the swinging motion. Then he had to only hold on with one hand so he could pick up the ring. Most importantly, he loved every minute of it. This worked on core strength, fine motor, coordination, proprioceptive and vestibular systems. My sons' days in OT were filled with these exercises.
My sons did not have fine motor skill issues, but I was present for both their evaluations and copying block structures is always included. I believe, but am not an OT so can't say for sure, the OT evaluation is looking at how the child manipulates the blocks with their hands and if they are able to copy the simple structures. It's different from an IQ test because the block patterns don't get that difficult. Perhaps she noticed your son struggling with one aspect of the test and suspected some visual hindrance.
I would agree with the previous post that you should get the vision evaluation to obtain a baseline and then see if the OT work helps that later on. I also always consulted our pediatrician when we were given advice from an OT or Speech person before proceeding.
One last comment, you know your child best. If you have any kind of uncomfortable feeling with your OT, I would look for another one. There are so many good ones out there. And, one OT who is perfect for one child might not be a good fit with another child. Good luck.
