Diagnosing bipolar in kids is not easy. One of the hallmarks of bipolar, the mood swings and especially the depressive stuff, goes hand in hand with a lot of other childhood disorders including ADHD and autism. These kids also can seem manic when they are very happy (for a change) often doing something they enjoy. Take away whatever it is they are enjoying, and you can see an amazingly rapid transition to rage which someone who doesn't fully 'get it' can interpret (wrongly) as bipolar. it is most important that all possibilities are carefully considered, and you make sure you haven't just taken him to a doctor who has a 'revolving door' approach to diagnosis (ie every patient who goes in his door, comes out with the same diagnosis, really fast like an assembly line. Fast turnaround, like the large revolving doors in some buildings).
I'm not saying the diagnosis is wrong, only that as with so many of us and so often with each kid, the diagnosis MAY be wrong. Again.
The ADHD may not be wrong. Or it may be. If he is worse now, it could be because ADHD medications (which may have been working) have been removed. All I'm saying, is keep an open mind.
The ODD stuff - it can be managed. Personally, I don't like the label, I think it is misleading and causes more distress than it deserves. Especially distress to parents, who are led to believe their child is choosing to be difficult, perhaps for the enjoyment of it. That generally is not the case. We in fact often teach our kids to be oppositional, by using the more conventional discipline methods. These work well on most kids, but on some kids, they are a bad fit. And you won't know until it's too late and the damage is done. But the damage can be undone, although it takes a change in mind-set. It helps if you have a good understanding of what is going on in his head, and it sounds like right now, you don't have that, with the change in diagnosis.
In summary - I believe ODD exists, but is not what some professionals claim it to be. And we do it to our own kids with a badly fitting discipline. A change in discipline method to something with more self-determination can turn this around. You need to step back from the "Because I said so" approach and towards something more like, "Let's work as a team on this problem." Mostly these children learn by imitation, and when you model controlling behaviour for them (which is what we all do, when we ry to use the "Because I said so" approach) then they will simply dish it back up to us, then be surprised when we react with anger at the "disrespect".
I think that behind almost every kid with an ODD label, is a problem with social skills. This can be generally found in autism, but in some other disorders too.
A lot of these problems can be dealt with by actively working with the child on these issues. You don't have to be a professional (although it helps to have one on board). We had to do a lot for ourselves because we are isolated. I only found out years later, why some of our successful interventions were so successful.
Welcome on board, stick around. And as the others said - a neuropsychologist evaluation might clarify matters. If nothing else, it can give you a sense of direction, and of things that could help him. And you.
Marg