I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
That is most likely due to oak leaf blister disease or a similar fungus. It generally doesn't re-attack the tree in the summer so I would not worry about it now. The damage was done when the leaves first were emerging although symptoms showed up much later. The following web page gives more information:
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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