OCD behaviors that come up suddenly...can be Pandas. This is an autoimmune reaction to the Strep bacteria.

Some OCD can be from nutrient deficiencies. Some from genetics (temperment), and some from Pandas. Autoimmune issues can arise from gluten intolerance/Celiac as well as malabsorption of nutrients.

Kids often do repetitive things to calm themselves, as well. So it might be hard to separate out that emotional component.

I have never heard of "food therapy"... can you explain that more?
Kids with sensory issues can become "pickey" about food, how it feels in the mouth etc. Sensory Integration issues are a whole disicipline in themselves.
Does you child get enough protein each day? Enough electrolytes?
This is important for growth.

Inositol is often used for mild OCD...this is a powder and tasteless and can be mixed into juice. But I would have a doctor supervise its use in a small child, and give the dosage needed for your child.