Heat oven to 350°F. Cook and drain pasta as directed on package. Return to pan; set aside.
Meanwhile, in 10-inch skillet, cook sausage and onion over medium heat 8 to 10 minutes, stirring occasionally, until sausage is no longer pink; drain. Remove from heat. Stir in oregano and Cajun seasoning.
Stir sausage mixture into pasta. Stir in remaining ingredients. Spoon into ungreased 2-quart casserole. Cover with foil.
Bake 50 to 60 minutes or until hot in center.