These mushrooms are probably one of the myriad species of fungi responsible for wood decay and nutrient recycling. Some actively infect trees as pathogens but many only passively take advantage of wood that is already dead or dying, such as those that continue to appear long after tree removal. The appearance of these doesn't necessarily mean they were responsible for killing the oak. Other mushrooms are mycorrhizal, which means they grow in partnership with plant roots, helping them absorb what they need in return for some nourishment of their own from the plant's energy stores. Most plants grow with one or more mycorrhizal fungal partners that help them grow and survive in tough conditions. Even on a recently-removed plant, the fungal partners might continue to survive for some time later, and many fungi produce mushrooms when conditions are right (often after wet weather).

Mushrooms are akin to the flowers or fruit of a plant, in that they are reproductive structures and their removal doesn't hurt the "body" of the fungus any more than picking flowers or fruits harms the body of a plant. Mushrooms can still reappear for some time depending on how long the fungus' food source underground lasts and how cooperative the weather is for mushroom production. While they can be valuable for wildlife (box turtles love mushrooms, for example) and for living plants (who can then use the nutrients the mushrooms release from the decaying wood), if you're concerned about curious children or pets trying to consume them, you can dig up or pluck off those you find before allowing them into the yard. Otherwise, the current crop of mushrooms will wither on their own soon enough. We don't comment on edibility since too many species can be difficult to discern from toxic look-alikes, and mushroom ID can be challenging even for experts; therefore, to be safe, do not consume any of them.

Miri