In 12-inch skillet, heat oil over medium heat. Cook and stir onions 5 minutes or just until softened. Stir in garlic; cook 1 minute. Stir in green beans; cover and heat over medium-low heat 7 to 10 minutes or until beans are crisp tender, stirring occasionally. Stir in undrained tomatoes, oregano, salt and pepper; heat to boiling. Spoon mixture into 2-quart casserole.