Preheat oven to 425 degrees F. Lightly coat a 15x10x1-inch baking pan with cooking spray; set aside. Clean mushrooms. Remove stems from mushrooms and discard. Brush outsides of mushroom caps lightly with olive oil. Place, stem side down, in prepared pan. Bake for 5 minutes. Drain mushroom caps, stem sides down, on a double thickness of paper towels. Drain all liquid from pan.
In a medium bowl stir together tomato, olives, bread crumbs, feta cheese, garlic, oregano, salt, and pepper.
Spoon the crumb mixture into mushroom caps. Place the stuffed mushroom caps in the prepared baking pan. Bake mushrooms, uncovered, in the 425 degree F. oven about 10 minutes or until mushrooms are tender and heated through. Makes 12 mushrooms.