Sick oak tree
Thank you for contacting Ask an Expert. I believe this is a leaf scorch problem; these tend to show up in the hot weather. You should probably get the tree tested to see if it is a bacterial leaf scorch or not. Bacterial leaf scorch is a lethal disease transmitted by insects, whereas other leaf scorch diseases are a result of environmental stress and are not likely to cause great harm to the tree. I am attaching an article about Bacterial Leaf Scorch below. Try to reduce stress on the tree during droughty periods by watering and mulching.
I hope this is helpful.