Very common on red oaks. Caused by wasps or flies. Old galls look woody; new galls are lighter in color and spongy. This gall is common to red oak species. The tree makes the gall in reaction to insect larvae living in its tissue. While galls generally do not cause a terminal damage, a heavy infestation can damage young trees that are just trying to get established. Regardless of tree size, galls can decrease the vigor of the tree. Unfortunately, there’s not a whole lot that can be done. The best you can do is to keep the tree as healthy as possible with plenty of water and maybe a little slow release fertilizer at the right time of year and the correct quantity.
(photo by Jim Baker, North Carolina State University, Bugwood.org)