Is your vet 100% certain its an OCD lesion? Its not in the most common sites for OCD of the stifle (assuming I'm reading the radiograph correctly?) If it is OCD, the best prognosis is for lesions that are less that 2cm long and less than 5mm deep.
But to me, that looks more like a subchrondral bone cyst (SCB cyst). Either way, it will need surgical debridement (cleaning up the bone and removing any pieces) as well as injection of the cyst with corticosteroids and a controlled exercise program. The prognosis can be as good as 75-85%.
However, you major problem with either of these two conditions is osteoarthritis/degenerative bone disease, especially as you want to barrel race which is hard on a horses joints. There are lots of joint supplements and treatments to try prevent/reduce the amount os osteoarthritis. Have you heard of IRAP? I'm not sure its in your country and it is quite expensive, but its shown great results with DJD.
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