It looks like Armillaria mellea, honey mushroom, a common root rot fungus. You may want to have an arborist take a look at your tree.

Here is a link that might be useful: {{gwi:369901}}

OP said they were sprouting from stumps so no need to worry about the health of the dead tree.

Oops; good catch; Mea maxima culpa...(-:

Armillaria is a pathogenic fungus species, attacking and infecting living trees. It might have been the cause of the demise of the oak. If these mushrooms are spotted anywhere near a living tree, it might mean that the fungus has infected the underground root system.

Before jumping to conclusions on what it is, it would be good to examine the underside of the mushroom and other characteristics to make a positive id.

Rest assured that the demise of the oak was from a chain saw!
I'm known as the mad oak killer up here in what used to be the pine forest.

One more type from this fall. This one was in ground and would push up through the soil. A bit larger than the other type and singular.