Looks to me like a species of Armillaria. The only thing that seems problematic is the thickness of the stem. Was there a ring on the stem? Did the mushrooms appear in clusters? IF so, it looks like it might armillaria - one of the few parasitic mushroom species and a real potential problem.

It's an Amanita species.They are a mycorrhizal genus and would pose no threat at all to your oak tree.It is actually to your trees' benifit.

No worries.