If one of my sons has been diagnosed with oppositional defiant disorder, what are the chances my other son will have it too?

Common: In my experience the most common reason for children to get diagnosed with odd, is undiagnosed sensory processing disorder. This is identified with a specialized type of occupational therapy eval. More structure and guidance would help you son, remember you are in charge and need to modle good behavior, do not continue to support conflict. Instead remember everything is an educational opportunity.

Thorough evaluation: Get a good evaluation of the symproms and diagnosis by a psychologist. Sometime it looks like odd, but it is really something else. I would also get a good physical work up by the physician. Odd is often trauma related. Has the child had a history of any kind of trauma or head injury? Be sure to bring this up at his evaluation. Then you have a better idea if the other child may be affected.