M.,
How does a child that young get diagnosed with ODD? I am amazed!
I have a couple of suggestions, but no direct experience.
Try for testing for aspberger's/autism too.
There are support groups for all of the above: ADHD, ODD, Autism. Find one on the internet. See if your town/county has one that meets. GO to the meeting(s) and just listen. There are many people who have been there before.
Remember, it is your son's story, not yours. :) Listen to him too. Make a little 1:1 time for all your kids. Even 5 minutes helps a lot.
Try: putting out your hands, palm up. Ask him to put his hands on yours (he may slap at first, that's okay, he's just got to get used to it). Talk to him in this position, too; cross-legged or on a chair directly across from him. Let him know he is the center of the conversation, and he has input too.
Low voice, no emotion except positives, and talk slow with good diction (clear, no contractions and on his level).
This is a start. There are a ton more things you could be doing, but this is just one.
I wish you luck and much joy in the future with your children.
M.