My son's doc explained it like this to me: He said when kids have ADHD they are get in trouble for just about everything they do! They are constantly being redirected, told "No", etc which makes a kid feel awful inside! He said the bad feelings come out in one of two ways.
1. Bad self esteem- They turn the feeling inward. This was my son. He would say things like "I'm not a good boy. I'm bad.", "The other kids don't like me because I'm always in time out", "I can't do anything right". And he was only 3 years old!
2. ODD-They turn the feelings outward toward others around them and it come out as very defiant behavior.
The ONLY thing that worked for my son was medication. Once we got the meds right his self esteem issues went almost completely away! I've also heard many say the same about ODD once the ADHD is under control. Good luck! 
Thank you for the input. We are starting concerta tomorrow after having not much luck with the ritalin sa.we shall see! thanks again.
Our son also has ADHD with ODD. I think we have settled on his medication- Vyvansse ER(morning) and Adderall IR (afternoon), as well as Fluvox for OCD/anxiety. With the ODD, it seems like it goes in waves... gets really extreme for a time and then settles into a medium/light cycle and then grows back to extreme after a while. I have no idea what contributes to this, but that's the way it seems to go with him.