Let's look at it this way -- if a medical doctor told you your son had diabetes and you didn't think this was the right diagnosis, what would you do? Right: You'd get a second opinion. And that's what you need to do in this case.
If you think your son has oppositional defiant disorder (ODD), this is a more serious diagnosis than ADHD and should be treated differently. You should ask your son's pediatrician to refer you to a child psychiatrist or a clinical psychologist who has experience with lots of kids with both ADHD and ODD. Tell that doctor that you need to have your questions answered and ask if he is willing to come to a meeting at the school to share his findings. If he won't, go to another doctor.
When you've found a professional who will help you make your case, ask for a meeting of the school folks. Also include the person who made the original diagnosis of ADHD and your son's counselor. Ask them to plan enough time to reach an agreement about your son's diagnosis. You want to leave that meeting with all parties "on the same page" about your son and his treatment.
Remember, though, if this objective third person (the new doctor) says that your son has ADHD, you have to be willing to place some confidence in that diagnosis. After all, that's why you're going there, right? If it is ADHD and not ODD, then it may mean that you need more support than you have right now to deal with your son. In that case, ask the school to help you find parenting programs or childcare in your home.
You say that the school doesn't understand your son's needs. Having an independent objective person at this meeting can help you communicate with the school. It may be that your son needs more help than the school can provide. He may need a special program in anger management, or he may need to be in a special therapeutic school for a while to help him gain more self-control. Finally, if you feel that your son's rights are being violated by improper or inadequate treatment, you can ask the Office for Civil Rights to get involved. They can sometimes add a voice of reason to a situation that's spinning out of control. The main thing in situations like this is to get all the players together and make a commitment not to stop talking until the issues get resolved.