Maybe it's just me... but when a kid comes to see me, having already been given an ODD diagnosis, I'm immediately suspicious. Why? In my experience, there's almost always another problem that better explains the child's behavior. I'll get into that in a sec. First, let's talk about the ODD diagnosis itself. Rather than list out all the criteria for the diagnosis, I'll just list the 3 core categories: Angry/irritable mood; Argumentative/defiant behavior; and Vindictiveness. These symptoms need to have lasted for at least 6 months for a diagnosis. I think this sums up the diagnosis well without getting too far into the weeds.
One of the problems with the ODD diagnosis is that it's tautological. In other words, someone has ODD because they're oppositional and irritable... why are they oppositional and irritable? Because they have ODD. See how that works? It's often the case that therapists giving a child an ODD diagnosis have missed less prominent symptoms that may better explain a child's behavior. ADHD, depression, bipolar disorder, and others can absolutely cause the behavioral and emotional problems seen in ODD... especially if these conditions are untreated.
Let's be clear though, ODD is a valid diagnosis. Is it often mis-diagnosed? Yes. Are there typically underlying issues that account for ODD behavior and emotions? In my experience, yes. ODD is what we in the mental health profession often call a "catch-all" diagnosis. It's too broad and vague in its criteria. Because of this, kids with other conditions that may mimic ODD symptoms are "caught" in the diagnosis.
The moral of the story is this... If your therapist discusses giving your child an ODD diagnosis, ask a lot of questions. Ask about other conditions that can cause the behavioral and emotional problems seen in ODD. Ask your therapist to rule out these conditions prior to giving your child an ODD diagnosis. After all, the initial diagnosis tends to drive treatment, and being on the wrong road can get you lost fast.