Galls are an abnormal swelling of plant tissue and can be caused by mites, insects, nematodes, bacteria or fungi. Galls usually are found on leaves and stems, but can sometimes be found on other locations of the plant.
In this case, the organism causing the oak leaf gall is a wasp. The gall grows around the insect and helps to protect it from predators and weather. The gall will also provide nutrients to the wasp until the wasp is fully mature.
The galls do not seriously affect the health of the tree, but they can be unsightly. Heavy infestations may distort the leaves or cause early leaf drop. Once the galls are on the tree, there is really nothing that can be done about them. When you have galls on the leaves, they will eventually drop off with the leaves.
For more information on galls, please see this publication.