They can co-exist, but most often ADHD/ODD are codeworks for early onset bipolar in kids, especially when mood disorders, substance abuse and/or suicide attempts are on either side of the family tree. Bipolar has a 90% ODD rate, higher than ADHD.
They are very different. Look at my post under your help thread, then look at the diagnostic criteria on this website for ADHD.DIFFRENCE FROM ADHD AND ODD?
See my response under your other post.