I really think that a lot of our approach to identifying people who need support would work better if we focused on smaller-than-diagnostic modules (such as social interaction problems), rather than characteristic clusters (like ADHD or ODD or ASD or whatever). This particular analysis seems to share that way of thinking about kids who need support.
I've been really wondering a lot about the overlap between ASD and ODD for years now, especially as we have tried to have playdates with kids with the same diagnosis as my kids, but a very different range of symptoms. I had convos back when A. was doing EI with therapists who said that some parents diagnosis shop, which I just wasn't sure whether I really believed that was a full explanation or not (really, are all these high energy, high conflict kids with ASD diagnoses actually ODD instead? Because they sure set off my ASD-dar for the rest of the package: social interaction issues, sensory problems, narrow areas of deep interest, etc.). I try not to blame parents (because I don't want to be blamed, either!), and I sure recognize that when a parent has needs that aren't met, that is going to limit how much the parent can then do for their child. Part of why we work with a play therapist is because I recognize my limitations at helping kids solve relationship problems in a play context without either creating high structure or making the situation worse in some other way (neither of which gets at the thing I want my kids to be better at than I am, which is being able to relate well to other people and share goals and cooperate and All That Good, Prosocial Stuff).
I'm starting to think that maybe ODD actually starts out as what I think of as Grumpy Old Man Syndrome. You say, hey, let's do X. GOM says, NO! If you wait a few minutes, GOM will say, hey, I have an idea. Let's do X! Annoying as fuck, but manageable, if you can avoid engaging with the initial no. If you make the mistake of trying to convince GOM to do X, GOM will escalate and entrench, until GOM is saying that X will kill the kiddies, give us all cancer and directly start Armageddon within his limited remaining lifetime. Also, cost too much money. A parent that doesn't have GOM, and who figures out early on to wait, and maybe do some environmental nudging towards X, and doesn't require the GOM to admit it was actually the parent's idea, etc. etc. etc. can take a kid with GOM and produce an adult who may be a little annoying at times (especially when tired or surprised out of their routine), but is basically functional. A parent who has severe GOM themselves may wind up making an adult with severe GOM, aka, ODD. Assuming they don't just kill each other when the kid hits puberty.
We tend to think, oh, let's fix the kids and then it'll be good in the next generation. And I'm like, yeah, that's probably never going to be good enough for things like really entrenched, multi-generational GOM. You are going to have to mitigate with the 'rents, if those kids are gonna have any chance at all.