Answer:
There are two "schools" of Occupational Therapy.
One school is the traditional, fine motor development (picking up coins, stacking blocks, etc.) and eye/hand coordination exercises. This is definitely not quackery but practices which improve one's life.
The second is a more "modern" approach, which involves things like brushing, swinging, special chairs or cushions, eye training, etc. This is most definitely quackery.
Any time someone can't give you specific studies (that's plural), from trusted sources, and with reproducible results it's probably quackery. Any time you hear someone say, "It won't hurt", run away.