I have a 5 year old with both and this is how I deal with him. And it is dealing with him because you will spend a couple of years rerouting his coping mechanisims. I've been working the baskets of The Explosive Child by Ross Greene, PHD. I was referred to this book when my middle son kept getting physiacally violent at school and at home.
It really does work because you're not nit picking his actions to death and giving him the impression that he can't do anything right. Then he will feel overly picked on and you will feel like you've created a monster. While the motives and triggers behind ODD could very well be trauma sometimes they are just because a child has found this to be very effective and you're just too busy with 3 other kids to actually handle it well. It's a lot of work raising one kid but adding 1-3 more can really put a strain on your own coping skills.
The key is to stay as emotionless as you can. Poker Face Mom. divide is bad behavior into 3 categories, the ones that if he's corrected he meltsdown and you're willing to deal with the meltdown just to protect his safety or someone elses, the ones that sometimes he will meltdown but often he can be bribed out of doing and the ones that you would like to see go away because they get on your nerves but they aren't disrupting daily/hourly life.
So you tell him the first list is the stuff that if he does he's going to be punished for and no matter what he only gets one chance to choose between the bad behavior and something that won't get him into trouble. Say he wants the toy back from his younger brother, His usual choice of action is just to push his brother down and sit on him while the baby screams and he pries the toy from his brother's clenched fist. You flat tell him that if he takes toys without asking or accepting no for an answer then he's going to have to lose his freedom and no matter what kinda fit he choses to throw that's it there's no way out of it.
The next list you tell him that you're going to give him the option of choosing a better way to deal with his urges. You're going to say you have a choice between doing this or that and doing this will cost you something, (losing his favorite items, one at a time, in a specific order) and your going to give him a second chance to make the decision by counting down from 5 or up to 3 if he doesn't make his choice quickly.
The last list, well, you're just going to have to let those go for right now. Once you get the first ones under control these will get better on their own.
Now stick to it cause he's going to push you just to see how far he can before you explode yourself. For instance, being noisy is on my 2nd list. I've given my middle boy the choice of being quiet twice already this morning and the third time I had to give him the choice I got louder than him, his younger brother and the tv. I know I shouldn't have yelled but I slipped off the program and acted in retaliation.
If you want to know most of the motives behind his behavior then you can use this book along with the Explosive Child to really get a good picture of a child's mental and emotional motivations but for right now you just need to be the one in control. http://lifematters.com/step.asp
Yes you can use one method for one child and another for the others and it is equal. Because these kids need to understand what authority is right now not when they get to the stage that the other poster used as her example. By the time they get to see her mother(?) the often have so many bad habits that the need medications to control their impulses. The Explosive Child will help you avoid that.
Now it's not going to control the ADHD but it will often help him cope with it when the medication has worn off or hasn't had a chance to take effect yet. Keep him on his medication all the time to keep the effective level up in his system, yes that means vacations and breaks too. And keep in mind that the medication can also lead to emotional outbursts, like crying and as a side effect those cannot be punished out of his reaction system. You just have to evaluate his reaction to see if it's motivated by getting his own way, frustration or just emotional side effects and react accordingly. This is when your "mommy wants to hug the hurt away" urges will come in the handiest.
Next I want you to be sure you're spending at least 20 minutes alone with him a day. Bath and bed time is the best for this. You can do it as a natural progression starting with the baby and every half hour you move to the next older kid. Start with a bath, read books and cuddle then tuck them in. It should be at least 20 minutes for each child.
Best wishes for a New Year with new strategies to cope.