Preheat oven to 400°.
Remove brown gills from undersides of mushrooms using a spoon; discard gills. Place mushrooms, stem sides up, on a baking sheet coated with cooking spray. Sprinkle with 1/8 teaspoon salt. Bake at 400° for 5 minutes.
While mushrooms bake, combine ricotta, garlic powder, black pepper, and remaining 1/8 teaspoon salt in a medium bowl, stirring well. Gently stir in the spinach and bell pepper. Divide mixture evenly among mushroom caps. Top evenly with tomato sauce and cheese. Sprinkle evenly with oregano. Bake at 400° for 8 minutes. Increase oven temperature to broil. Broil 1 1/2 minutes or until cheese is melted.