My son was in OT for about 4 years. It took a few tries to find an OT who understood his issues, could establish rapport with him, and manage not to be manipulated or distracted by him when he was trying to get out of doing the difficult thing he didn't want to do.
From what you say, the OT will probably give you a "sensory diet" for your child, a list of activities to do at school and at home that will help your child develop better awareness and modulation of sensory input, and, given the handwriting issues, may possibly also include exercises to develop proximal stability, and strength and motor control. It might seem like all the OT is doing is playing games with your child, once the initial evaluation is over, but, ideally, all of the activities have very specific purposes.
We saw huge improvements in all of the areas you discuss, but I do have to warn you that, in our experience, frustration thresholds dropped dramatically just before the greatest gains were being made on a pretty consistent basis. Don't assume that because behavior deteriorates that the OT "isn't working".
