Gall wasps are a common problem here on the east end of Long Island. Oaks are a dominant species around here, and many clients rely on them for shade around their houses and yards. What should be done about trees that are found to be infested with gall wasps?
I've found that trees can put up with a lot. Oak trees are seldom affected by galls unless there is a heavy infestation. Galls on the leaves are not a worry, and oak trees will tolerate them quite well. You can rake up and destroy the leaves when they fall, if they are causing aesthetic issues. Pesticides are rarely effective. The kind that can be damaging is the galls that form on the branches. These aren't a problem if they don't get out of hand (there were only 3 cases I can think of where I actually had to do something about them), but if they seem to be in damaging quantities, prune out and destroy all galls as you see them form. If you get them before the wasps emerge, this will help control them for the next season. Continue this practice until the infestation is under control.
Spraying is expensive, and not very effective (you only get the wasps that are flying - in the earlier stages of growth the insects are protected by the galls). I don't recommend it. It is possible for an infestation of gall wasps to eventually kill a healthy tree, but in small numbers, these insects are not a big problem.
Here is a page I just found that supports the above information and has lot's of good detail in matters concerning gall wasps.