I always thought these brown, woody balls often found on oak trees were oak apples but I recently acquired a book on Britain's Plant Galls (Michael Chinery - ISBN 978-1903657-43-0) from the WildGuides series and I was surprised to discover they are actually the remains of the marble gall, formed by the larva of a small gall wasp (Andricus kollari).
Whilst the biology of these gall wasps is interesting it is also complicated and, if you are interested, then look elsewhere for the details; it would take too much to write it all here and, in any event, Michael Chinery knows far more about it than I do!
Sufficient to say that these balls form to provide a home for the wasp's larva. They are usually laid in small clusters and start out green gradually turning brown as the summer progresses. They become more visible, of course, when the leaves go from the tree. This species was introduced in the 19th century as a source of tannin for dyeing and ink. I didn't know that either, thanks Michael.