The disease is very serious in that it is the growth plats so affected. The abnormal growth of articular cartilage is usually coupled with epiphysitis which is over growth of bones at the growth plates (this is a VERY simplified explanation).
By the time the horse is lame or shows signs of OCD it is too late. The horse's joints are afflicted and often cannot even be pasture sound.
Over feeding young stock.. too fat young stock.. mares that milk like cows and so forth as well as genetics are all thought to be factors. Follow this link: When Joints Fail: Osteochondrosis | TheHorse.com
Get xrays. If the disease is obvious, do not take on this problem. The end is the same (a dead horse). Here is a photo of a cadaver joint showing the lesions in the cartilage of the weight bearing articular surface.
The other photo is a hock so affected. Looks a little boggy.. and it will get worse to the point where the horse may well be in so much pain he is unable to stand.