I don't know the particular name of the shroom, but yes, mushrooms live on dead/dying wood, and Oaks have specific ones that live on their dead/dying root material, such as the edible Señorita.
It is not necessarily a harbinger of doom, but removal of turf, soil aeration, and a root collar inspection would go a long way to ID'ing and perhaps alleviating the problem.
-Tom
(PS - Posting this over in the 'Bugs and Crud' forum might get you some better responses as well.)