Sounds like slipped hock, which is a loose achilles tendon. That can be really hard to fix if the bone channel itself is too shallow due to poor genetics. Some brace and let the chick grow, but if the channel is simply to shallow, the tendon will just slip out again.
If it is due to mineral/vitamin deficiency, treating with supplemental feed or vitamin/mineral mix can cure it if caught in time, and it does sound like you just picked them up, so it is worth putting them all on Chick Saver. (Manganese is a key element). However, if you have been feeding regular chick start, it is doubtful it is from nutritional deficiency but rather genetic malformation. Time will tell.
I am of the farm mind so that I cull chicks with resistant slipped hock as I find the bird becomes disabled as it grows and places more weight on the leg. The leg will splay out or back at odd angles, and the bird is in obvious pain as it tries to hobble on the defective leg.
Somebody may have some recommendation for a more permanent bracing system. It will depend on how you feel about your birds and how much effort you want to put into this especially since an Orp is a heavier bird type which will place more stress on that hock as it grows.
Sorry I don't have a cheerier perspective. Slipped hock is difficult to fix if nutritional supplementation is not sufficient.
LofMc