Oppositional defiant disorder is a label only used with children. It's not a side-effect of ADD, but a certain percentage of children with ADD are also very oppositional. The common drugs were effective, but caused physical ticks. You should consult a physician. Eyer, go ahead with your question please.
Caffeine is a stimulant which does help ADD symptoms.
Lots of adults self-medicate with caffeine. However, caffeine is less effective and much harder on your body than taking stimulant medication prescribed by a physician.