This looks like oak leaf blister, a disease caused by the fungus Taphrina caerulescens. Infection occurs in early spring as buds open, and light green, blisterlike areas develop on leaves as they expand. As the growing season progresses, the blistered areas eventually will turn brown. Once symptoms are apparent like this, fungicides are not effective. We do not recommend any treatment for this disease on landscape oaks because the impact on the health of trees is minimal. Fungal diseases ebb and flow from year to year, depending on the weather conditions, so you may or may not see symptoms in future years.

Here is more information about oak leaf blister disease, from Ohio State University.

Christa