Preheat oven to 400°F. Remove stems, finely chop stems to equal 1/2 cup and reserve. Place mushroom caps in a bowl and toss with olive oil. Arrange caps cavity side up on a shallow baking pan and set aside. Heat oil remaining oil in skillet over high heat. Add sausage, green onion, garlic, Italian seasoning, and chopped mushroom stems. Saute, 5 minutes. Remove from heat. Add both cheeses and combine. Stuff sausage mixture into mushroom caps. Bake 10-15 minutes until mushrooms are tender and cheese is golden.