I saw this on my Pedunculate Oak tree.   Its formed when a female knopper gall wasp lays eggs on the acorn cup injecting a substance that triggers the tree to produce the galls.
Knopper galls are typically knobbly,  bowl shaped protrusions that start sticky and red and later turning brown and woody. 
The developing wasp larvae feed within the gall and the affected acorns become infertile.
(Thanks Wikipedia)