Red fuzzy balls on Pin Oak leaf
It appears that you have Fuzzy Gall that was formed from a gall wasp. The wasp actually "stings" the tree and within this is the larvae of the wasp. It is not detrimental to the tree.
If you have any other questions feel free to call us at 859-356-3155.
Kenton County Agent for Agriculture and Natural Resources
You are welcome. Have a great day.