Strange round thing attached to an oak tree branch

Oak Gall Wasp - the larva is nurture within plant tissue that grows around the eggs to protect the larva as it matures. (Courtesy of Paula Richards)
No, it’s not a fruit, it’s not an acorn…
(Courtesy of Paula Richards)

While taking a hike, we notice several of these small, hard nodules on various oak trees.  Some had small holes in them.  Can you guess what it is?

Click this link to find the answer.