what kind of oak??.. evergreen or deciduous?

Cercospora Leaf Spot is very common in the fall with hydrangeas. For hydrangeas, overhead watering promotes the spread so try watering the soil and never the leaves. You can apply fungicides on the hydrangeas but, since the leaves are about to fall anyway, discard leaves & blooms in the trash. Really badly infected leaves can be simply cut off.

its a deciduous oak.

thanks. we did have a wet spring and summer so maybe thats the cause. didnt know if i needed to do anything specific to help eliminate it