A friend of mine had done several clinics with Buck Brannaman and was quite a devotee of the ORS. He taught his little QH this manuever well (as per my friend anyway).
At another clinic he offered his QH to another rider to use. She did the ORS at a trot in the indoor arena and the horse went down, breaking her leg.
We all know that the movie stunt people used to drop horses by pulling their head over at speed. The ORS is supposed to be different in that the horse “disengages” his hindquarters, which means he crosses one hind leg in front of the other. It doesn’t take Einstein to see the potenial for a misstep here – especially in horses that might not be as well balanced, athletic or badly ridden.
So yes, I would say the ORS CAN be dangerous.
Yea, once my gaited pony bolted in the arena. When trying to do a one rein stop I lost my stirrup and I fell into the turn crashing hard on the ground. I think I would have been alright if I didn’t try to bend him but hind sight’s 20/20. 
I can see the potential for accidents. The way I’ve learned it, the reaction of the rider to begin the ORS (lifting the reins in one hand, reaching forward to get a short rein to bend the neck) is quick, but the application of it (pulling the head around) is done judiciously, depending on speed and the horse’s reaction. Because my horse and trainer and I practiced the sequence (lift-reach-pull-horse gives-release) many times at the halt and walk, she feels the beginning movement, gives her head slightly to the side I’ve reached on, and stops dead. Because we’ve rewarded and rewarded for giving the head and shifting the weight back and halting, she anticipates that, knows what to do when I begin to ask, which I think is more the point than just applying it in an emergency and hoping it works. At higher speeds, such as practicing at the trot/canter, the pulling around of the head is done with feel, even sometimes more like a series of pulses, responding to how the horse gives and is slowing down, so you don’t throw them off balance. Has anyone else learned it like that?
