Amy,
According to Mark Bullington (I'll put it all on him...), the person in charge of the horse show has said they will have AMHR classes. The website is yet to be updated, though.
The OSF is a great horse show, with some ADS carriage classes as well as a vast array of breed classes. The atmosphere and camaraderie in the horse barn and classes is wonderful...a true, old-style fair setting in the dog days of August/early September. I've had such a great time in the past and I really hope AMHR is back and that those classes have many entries. It's very close in time to Nationals, but if you're not heading to Tulsa, be sure to come! We'll definitely be there.