I have a mature oak tree in my yard. Every branch has a multiple growth of what looks like a hard walnut, with prickly things all around it. The size of a silver dollar. Does my tree have a disease of some sort.?
From your description it sounds as though your oak tree has galls. There are dozens of types of galls that affect oak trees; some attack only certain oaks. You don't indicate what type of oak tree you have, so it's impossible to tell from just a descripton which type of gall that your tree has. Galls may be caused by either insects or by fungal diseases.
Most oak galls are strictly cosmetic. That is, they may look bad, but cause no real harm to the tree. A few can cause twig or branch dieback if they completely encircle the branch. You shouldn't need to do any treatment other than to prune out dead portions of the tree during the dormant season.