First I want to say that most of the time a child with ODD also has ADHD...they are comorbid which means they go together/coexhist. Then I will tell you that for a time my son who has severe ADHD (and most likely ODD but hasn't been officially dignosed yet) took Clonidine at night to help him sleep. That was all he took it for.

My son took Clonidine for his ADHD for a while. It helped alot with his hyperactivity, but did nothing for his concentration (and may have even made that worse). Clondine can be quite sedating.

The problem with taking it by mouth is that it is short-acting. It only lasts about 3-4 hours, I believe. So, your son may have something of a rollercoaster effect from it. There is a patch form which my son used and it worked quite nicely. The only problem he had on it was severe constipation.