Wash mushrooms, remove stems, hollow out and brush inside and out with olive oil.
In a medium-size mixing bowl, combine cheese and pesto, reserving 1/4 cup Parmesan cheese for later. Stuff the cheese-pesto mixture into the mushroom caps. Arrange the caps on a cookie sheet. Sprinkle the mushrooms with the reserved Parmesan cheese.
Bake for 25 to 30 minutes, or until the cheese is bubbling and brownish.