Hi, Plexicos...
I have two hard-to-diagnosis kids who are now teenagers. Just some random thoughts that might or might not add to the discussion...
1) ODD is not a useful diagnosis, except to acknowledge that a child has major problems and that there is something serious that needs to be investigated. We had it for a year or so as a "placeholder" diagnosis. There are no medications or therapies that I know of that work for "ODD" as such.
2) Dxes are "boxes" and our kids are not "square". If someone fits the definition for a "box", they can more easily get a diagnosis. But... lots of our kids are a bit of this and a bit of that. Even so, comprehensive testing helps. For example, someone can have Autism Spectrum Disorders (ASD)/Aspie "traits" and still "not meet diagnostic cutoff", which means... some of their behaviours and thought patterns are best looked at through the "Autism Spectrum Disorders (ASD)" lens, and strategies that work for Autism Spectrum Disorders (ASD) people should be tried. There can be bits of various MH issues, developmental delays on any of several fronts, neuro-motor challenges, learning difficulties, and so on.
3) Family tree medical history, and prenatal exposure to various substances, have a significant impact on the make-up of every child. Some of these issues can be handled with knowledge, professional help and medications, others are just hard-wired (such as Fetal Alcohol Effects (FAE)/Fetal Alcohol Syndrome (FAS)).
As others have said, it REALLY pays to get a comprehensive evaluation...
As a side-note, though, if this kid does well on Ritalin and horrible when it wears off or is missed... why isn't he at least on extended formats (like Concerta) and/or 24-hour ADHD medications such as Strattera? We found the 24-hour format to be a huge boost, as it gives a "baseline".