Cook pasta according to package directions. Drain.
Meanwhile, in a large saucepan heat margarine or butter over medium heat. Add garlic. Cook for 30 seconds. Stir in flour. Add milk. Cook and stir until mixture is thickened and bubbly; reduce heat. Add shredded cheeses. Stir until melted. Remove from heat. Stir in cooked pasta and oregano. Spoon mixture into an ungreased 1-1/2-quart casserole. Top with bread crumbs.
Bake, uncovered, in a 350 degree oven for 20 minutes. Sprinkle with pistachios and crumbled cheese, if desired. Bake for 5 minutes or until bubbly. Let stand 10 minutes before serving. Garnish individual servings with green onion tops, if desired. Makes 6 servings.