Welcome. Sorry it's such a struggle for you. There are a lot of factors here which could be complicating the picture. This could be more than ADHD. As for ODD, I won't go there - I hate the term, I think it misleads people into blaming the child, or even blaming the parents.
A major factor in a lot of problems we see, is how we handle the kids discipline-wise. With most kids, the more conventional parenting methods are fine. They work. But with some kids, they backfire badly and you have to change tactics. If you use those changed tactics on other kids, they still work.
A good book to get your hands on is "The Explosive Child" by Ross Greene. It helps you get inside the heads of these kids and respond to them in ways they can accept and follow. Never lose sight of your ultimate goal - to get this child to a healthy, functioning adulthood. We sometimes can get lost along the way.
One starting point behaviour-wise - to teach the child respect, you first have to demonstrate respect to the child, even if you feel they do not deserve it. As Dr Phil says, "someone has to be the hero, and it may as well be the adult who has to lead the way."
This sounds like "letting the child get away with it" or, as my mother in law says, "letting the child win". But if you have the attitude of this being a constant battle of will,s you have already lost the war. You need to become your child's facilitator and support, not the obstacle. And if all your child ever hears form you (even if with reason) is "No!" then the child will see you as an obstacle, and will never get the connection that actually, it is not safe to run across the street in front of a truck and no, it is not a good idea to rush out into the snow without a coat on. But there are other ways of achieving the same goals without being an obstacle. Giving the child the choices where possible, especially where really, they don't matter to you - it helps. I wonder if these kids have felt that their lives are being controlled and directed by others and that the "ODD" child is actually beginning to kick back and try to wrench control back. Of course a child without supports is less capable of making sound choices, but with you as a guide to help, it is a valuable skill they can actually learn earlier. In the process of learning this, the child can then realise that you are there to help and not hinder, then they begin to turn to you for advice and support. This is what you want. But it can quickly snap back to old conflicts if you relax your self-vigilance.
Read the book. I found for me that simply reading the book brought about improved behaviour in my child. At some level it must have been changing how I responded to him, but I didn't think it was. Or perhaps it was changing how I perceived his behaviour. Anyway, especially after I began to put the techniques into practice, we saw a lot of improvement. However, the biggest conflicts were between difficult child 3 and husband, and it doesn't take much for them to be in conflict again. husband has made great strides in how he handles difficult child 3, but it is difficult for him. And difficult child 3 can be very reactive, also feels very entitled sometimes.
With chores, what helped us (still does) is to work as a team. If difficult child 3 sees me working alongside him, he works more willingly. I can also usually engage him with "If you help me do X, I will help you do Y." And of course while you work together, you are also modelling for him how to do a certain task, and he is learning. For example, I teach my kids to cook, to sew and to do laundry. Especially their own laundry. But if a job seems insurmountable, often a child with ADHD needs extra help to break up the task into manageable bits.
A child with a keen sense of injustice, with attention issues and a history of feeling shoved aside will be carrying a huge load of resentment and suspicion. All this is on top of any other issues they may have.
So make a list of what you want to fix. Then look at the list again - cut it back to three things. Ignore the rest for now. Utterly. And also remember - that list is not "We MUST fix it as a top priority come what may!" ALWAYS back off before a meltdown. Learn to recognise his warning signs and never force an issue. This sounds like giving way but in fact it is teaching him that you are trying to lead him, not drag him. When he is calm you can re-visit the issue. If you keep running into a brick wall with it, you need to change direction or change issues you're dealing with.
Example - respecting adults. I found difficult child 3 did not do this because adults were not respecting him (from his perspective). So I changed how I dealt with him. Observe yourself - how do you deal with this boy? We do tend to give our kids orders, even if we couch them with "please" (which we often do not do). But for some kids, they will dish back to others, the way those others have treated them. If your son is doing this, then I would strongly urge further evaluation because it indicates a deficit (maybe only mild but it doesn't take much) in social understanding.
When we started showing respect to difficult child 3, he began showing respect to us. But one of his teachers would not take the message form me, would not listen and continued to do (as she did to all her students) the whole sarcasm control thing. And so to her, difficult child 3 gave back the same behaviour she showed to him. You cannot punish disrespect in kids like this, because it is anger, frustration and a sense of justification that is driving it. Punishing this only causes more resentment, anger and sense of justification. Instead, you pause and say, "Let's calm down now and look at this. You just said 'X' to me. Did that help me understand what you wanted? What would be a better way for you to get from me what you want? How about if you had said 'Y'?"
Staying calm works, but it is not easy when the child has made us angry. But if we respond to our anger at the child by yelling at the child, all we do is model for the child how to respond in anger. What we need to teach the child is self-control, and first we have to model it. These kids generally learn by imitating us, and if you watch your own behaviour, you may be horrified at yourself. But don't let guilt get in the way - this is normal. We learn to parent, by following the examples we were taught by our parents. And they did not have good books, they just had the methods THEY were taught. And as I said, these methods work, for most kids. But not for all, and when these methods don't work, the failure can be spectacular, as you are finding.
Read the book. it helps.
Welcome to the site. Many of us have been where you are now, and are happy to share what worked for us.
Marg