Lots of interesting thoughts here, so I'll give my two cents as well.

First of all, I sailed and raced the Opti for two years. And as many of you correctly has pointed out, the boat is very slow, it has a very poor hull design hydrodynamically speaking, and a low tech rig. The boat is also impossible to sail two up. However, my experience from sailing it is that exactly because of the poor hull design and the low tech rig the boat gave very instant and direct feedback when you did something wrong or right, thus making it a very good learner boat. Because of its bathtub chaped hull it required a very, very good feel when riding waves, both upwind and downhill. This is of course important in any sailing boat, but especially important in the oppy as you truly did kill your speed if you didn't do it right.

As already mentioned, the rig made the boat possible to handle in hairy conditions. With a kicker and a spri-pole you easily learned the basics for sail trim. The sail design also required that the sailor could master the technique of "riding American" (don't know if that's a Norwegian expression) which means you heel the boat to windward to get the top of the sail as high as possible in light conditions. As the hull is rather boxy in shape this required quite a bit of training and balance to master well. In rougher conditions the boat would plane on the waves, making it a wet but very fun ride. As in multis, in these conditions you had to be careful not to stick you bow in. This is also the case for other dinghies, but owning a Laser myself I know that it is far harder to keep an oppy on plane than the Laser, as the Laser's hull to a certain degree does the job for you.

So to me it seems there is a bigger problem finding a boat that kids can take up after the Oppy. Here in Norway most sailors go from the Oppy to the Europe dinghy, which in short is a milder version of the Laser, but very expensive.