It appears that your oak leaves are being attacked by oak lace bugs.

During the weeks of summer, the symptoms caused by oak lace bugs can be a concern to some landscapers & nurserymen. Often, the infestations are most pronounced on white oaks. From a distance, the stippling of leaves from this piecing-sucking insect can produce symptoms that are like those from oak spider mites or classic leaf scorch. Closer inspection determines the identity of the pest. Be certain to examine leaf undersides.

With
an additional generation or two, by late August the leaf feeding
symptoms can be more severe. (Photo Credit: Steven K. Rettke, Rutgers
Coop. Ext.)

With only the 1st generation active in early July, the feeding symptoms by
the oak lace bugs are still mild. (Photo Credit: Steven K. Rettke,
Rutgers Coop. Ext.)

Although the leaf discoloration of the foliage caused by oak lace bugs can be alarming, spraying is generally not considered to be necessary. Tall landscape oaks would require large spray volumes of insecticides to achieve controls.This pest primarily creates an aesthetic concern. It is doubtful the insects have significantly reduced the tree’s ability to store starches & sugars during much of the summer. Infested trees are often able to perform adequate photosynthesis before symptoms become severe. The plant health damage to large oaks in the landscape is usually minimal.

My apologies...I sent my response before finishing.

If you find no insects, your tree is probably infected with a fungal leaf spot disease. Most leaf spot diseases are very host-specific, meaning they will only affect one type of plant or a related plant family. Generally, a disease
is not going to move from an oak tree to a maple tree or any other unrelated trees.

Fungicide applications are not necessary because the leaf spots are rarely severe and the leaves can still function. Although infections may be extensive in some years, little damage actually results. This is one of those plant diseases that appears worse than it actually is. Fungicide applications won’t cure the disease once the spots have formed on the leaves.

Also, from a practical standpoint, fungicides are difficult to apply to large trees. For small trees, fungicides containing mancozeb or
chlorothalonil can be applied to prevent spotting, but must be applied prior to and during bud break, when dormant buds open and shoots begin to grow, in early spring. Once bud break has occurred and symptoms are visible, it’s too late to spray.

Leaf spot diseases will not kill a tree that is otherwise healthy. If a tree dies during the summer, it’s more likely because of drought stress, root damage or other issues not related to a leaf spot disease. Keep trees healthy by providing supplemental water during extreme drought conditions, adding mulch to the root zone to conserve water and avoiding mechanical equipment injuries to the roots or trunks.

Although this is unsightly, I still believe there is no action you need to take. Upon further research, one other possibility is jumping oak gall. Again, no action needs to be taken. Galls are odd looking formations that are caused by tiny insects, usually very small wasps, that feed on the juices of the leaves (sap). They protect themselves by forming some sort of covering such as you see on your leaves. Here is some information on jumping oak gall:

https://www.canr.msu.edu/news/jumping_oak_gall_causing_damage_to_white_oak

Again, I do not believe you need to take any action.  The galls may not appear next year. The best thing you do is to rake up and destroy fallen leaves this fall (do not compost). This will help prevent a re-occurance next year. Trees affected are usually those that are not maintained (like your "volunteer" oak. Try to keep your trees healthy with sufficient moisture and nutrients.