HEAT oven to 425ºF. Spray a 13 x 9-inch baking dish with no-stick cooking spray.
HEAT oil in arge skillet over medium heat. Add onion, garlic and oregano; sauté 2 minutes. Add diced tomatoes with juice and black olives; bring to a boil. Remove from heat.
PLACE fish in prepared baking dish; season both sides with salt and pepper. Spoon tomato mixture over fish. Bake 10 minutes or until fish flakes easily with a fork. Remove from oven; sprinkle with feta and parsley. Serve.