Here in VA, Medicaid operates sort of like an HMO. You have to go to a "preferred provider." I haven't seen the list for psychiatric, but my daughter has had no problem finding doctors for her son that take Medicaid. If NC is similar, do you have a booklet listing participating providers? Definitely request one.
I would go back to the psychiatrist and push for a more thorough evaluation, and go in armed with a list of questions. ODD is sort of a catch-all diagnosis, in my opinion ... many times there is an underlying disorder. Keep in mind too, that early onset bipolar disorder has many of the same symptoms as ADHD. You might ask your psychiatris's opinion of EOBD .. since there is still a contingent of doctors out there that don't believe in it. I'm not suggesting your son has this, just that perhaps it should be on the list of things to rule out. If you have a psychiatrist who doesn't believe in it, it'd be a red flag of a narrow minded doctor .. again in my opinion.
Welcome!!