I can only cover the basics, since my experience is in the west.
That pic looks like decay of the heartwood. The center of the tree is normally dead and decay of that wood doesn't hurt the tree except to make it weaker--and it takes a very large decayed area to make if significantly weaker and subject to breakage. Scrape around in there with a knife blade and it should be soft if decayed. It could be evidence of some kind of root disease I haven't seen, however.
Bugs kill trees in 2 ways, by defoliating them or girdling them. Defoliation once or twice generally doesn't kill a tree but weakens it's resistance to other insect attack or root disease. Other insects are generally bark beetles which burrow under the bark and girdle the tree. If bark beetles you should see little holes in the bark with their burrowing dust below the holes. If you pull off the bark you'll see their pathways and the pattern of the tracks is unique to each species of beetle.
Root diseases spread thru the air or into a healthy tree when one of it's roots growing thru the soil bumps into an infected root. Take an ax to a sick tree and scrape off some bark down near the ground. If it is a root disease you are likely to find fungal growth under the bark. If you don't find it low on the tree dig along the major roots, scraping off bark looking fungal growth. One of the most common root diseases that can infect almost any species is Armillaria and it produces clusters of honey colored mushrooms above the roots and a fan shaped white fungal growth under the bark. Search Google images for Armillaria for example pics. Armillaria attacks trees most often when the trees are stressed--drought, soil compaction, etc. But there are probably several other kinds of disease that could affect your oaks too. Root diseases can decay the roots and they kill the tree by killing the tissue beneath the bark which girdles the tree. Root diseases often affect trees in patches when the spread is root to root and the solution is often to cut all the affected trees including a buffer around the affected area and to switch to a different species. If it's a tree here and one over there situation, something else is going on.
I suppose there could be infections farther up the tree that could also girdle it, but I haven't seen any in my area. And if so, I would expect some species to sprout from the root collar below the infection.
I am not aware of any fungicide you could use.
