In a large skillet heat olive oil over medium high heat. Add the onions and cook until tender, about 2-3 minutes. Add the garlic and cook until fragrant, about a minute more. Add tomatoes, mushrooms, olives, oregano, salt, and pepper. Bring sauce to boiling.
Gently place the fish fillets in the pan and scoop some of the sauce over the fillets. Return to boiling. Reduce the heat and cover. Simmer for 8 to 10 minutes or until fish flakes easily when tested with a fork. With a wide spatula, lift fish from skillet to a serving dish. Spoon sauce over fish. Serve with rice, and/or crusty bread.