We also have a few oak trees, which I've always considered to be among the best of the large trees. Since we moved to our house in 2011, these oak trees have had galls on them every year. We had a certified arborist take a look at them a few years ago, and he told us that the galls usually do not affect the health of the tree. They are caused by wasps that lay eggs in the branches, causing them to produce the galls.
This year, however, our oaks trees are so heavily infested with galls that I am concerned about the health of the tree. You can see the many galls in this picture and how the twigs die beyond the gall.
Here's a closer look at the gall itself (at least the most prevalent type--we have several types: horned, gouty, and apple gall).
So I called the arborist back to re-assess the condition of the trees, some of which are very near our house. He told me that he has seen an increased amount of gall infestation this year and that an effective control for the wasps producing this damage has not been determined. For now, his recommendation was to rely on biological control, that is, cleaning up the galls that fall around the tree and letting the natural predators of the wasps and their larva do their job. Since the trees do not appear to be in imminent danger of dying, we decided not to remove any and hope for improvement next year.
More information on oak galls can be found here,