Cook spaghetti according to package directions. Meanwhile, in a large skillet, saute mushrooms and green pepper in butter until tender. Stir in flour, oregano, salt and pepper. Dissolve bouillon in water; gradually add to skillet. Stir in milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Add turkey and pimientos.
Drain spaghetti; toss with the turkey mixture. Pour into a greased 11x7-in. baking dish. Sprinkle with Parmesan cheese. Bake, uncovered, at 350° for 18-22 minutes or until heated through.