These appear to be galls, which are tumor-like growths on plants triggered by an organism like an insect, mite, or sometimes microbes. In this case, these look like Horned Oak Galls or Gouty Oak Galls, which are created by wasps (not a type that bothers people, so no stinging). Both the Missouri Botanical Garden and Ohio State University have nice pages [see links] with images that describe and illustrate the unique and complex life history of these native insects. No action needs to be taken (nor would insecticides be very effective anyway, even if they were warranted) and the tree will probably not be significantly harmed.

Miri