Your tree sounds like a wonderful asset to your property; white oaks are important trees in our ecosystem. 

From the pictures I found in researching your question, the damage from this wasp-induced gall can appear to be very serious, even defoliating the tree.  The good news is that there is no real need to chemically treat the wasp; the damage may be severe for a year or two but a healthy tree will survive without serious injury.  Healthy trees are pretty good at growing new leaves.

Chemical treatment for jumping oak gall is not recommended.  IPM (Integrated Pest Management) practices include making sure the tree is healthy by providing adequate mulch, water, and spring fertilizer if needed.  Avoid damaging the trunk with lawnmowers, etc.  One management tool you do have is removal and disposal of the fallen leaves so that the wasp population doesn't build up too fast.  There can be two generations of wasps some years, but removal of damaged leaves should minimize the damage.

The links below are university-based information on the management of jumping oak galls.  Your tree should be fine.

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/other/galls/jumping-oak-galls

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

https://blogs.ext.vt.edu/henrico-hort/2018/06/01/from-the-henrico-extension-mg-help-desk-jumping-oak-gall/

All fine and dandy if the tree is in good health and responds organically.  My massive and majestic tree is probably marginal in terms of health.  Not many 150 year old trees are in great health.  They are more vulnerable to environmental extremes and pests.  Virtually every leaf on this tree is affected and regrowth is very minimal.

Can I spray a pretty powerful insecticide (Sevin?) under the tree in early spring to kill emerging wasps? If so, when?  Thanks

Jim Zablocki

All of the 5 references I have found (from different universities) state that no chemical treatment is recommended or needed. The link below has that same opinion.
https://hortsense.cahnrs.wsu.edu/fact-sheet/oak-jumping-oak-gall/
University of Kentucky, in the link below, has this paragraph that suggests a treatment if you absolutely felt you had to use chemicals.  Notice that it says large trees would require professional equipment; it is not likely you could do an effective job by yourself.

"Leaf Galls

Leaf galls rarely affect tree health so control is rarely justified. However, an application of carbaryl (Sevin) at bud break may reduce infestations of some galls. It is difficult to spray moderate to large trees without special equipment and the necessary protective clothing to protect the applicator from spray drift."

https://entomology.ca.uky.edu/ef408

I would recommend you contact a certified arborist if you want to chemically treat your tree.  Based on my research, I don't think it is likely to do much good and may harm non-target species.  If you do spray yourself, be sure to follow the label instructions carefully.  The label is the law.