For clarification, I'm assuming when you say OCD you're talking about Osteochondritis Dessicans, not the dog checking the locks multiple times and making sure his chew toys are alphabetized. I looked up some information on OCD and it seems to be somewhat hereditary, although I couldn't find a whole lot explaining exactly what causes it at the moment. Other things that are thought to be contributing factors to OCD are listed as: Too much stress on a young dog's bones, restricted blood flow to the cartilage, diet and nutrition, and weight problems may also be factors, but exactly how much or how preventable the condition might be isn't known. I had a shepherd and both parents and all 4 grandparents had their hips OFA'd at no less than 'Good' but out of his whole litter there was only one dog that had hips decent enough to get a 'Good' rating. The rest were worse... severely. So, health testing the parents is a great practice but isn't a clear pass to healthy puppies. Unfortunately, $h!7 happens sometimes. As far as your other questions on the situation, did you have any type of contract in conjunction with the sale of the puppy? Was there any kind of health guarantee on the pups? If the parents were tested and shown to be healthy, many breeders will guarantee the puppy's health, usually offering price of the dog to be refunded or a replacement dog if the pup turns out to have a congenital problem. Unless there's anything written about the good health of the puppy, from the information you've provided, it doesn't seem that your legally obligated to pay for anything.