Preheat oven to 400º. Stir together cornbread and bread cubes in a large bowl. Melt butter in a large saucepan over medium heat. Add sausage, and cook, covered and stirring occasionally, 5 to 7 minutes or until butter turns a light orange. Remove sausage with a slotted spoon; drain on paper towels.
Add onion and next 2 ingredients to butter; sauté 8 minutes or until tender. Add sage and next 2 ingredients, and sauté 1 minute. Stir in Cajun seasoning and next 2 ingredients; remove from heat.
Stir vegetables and sausage into cornbread mixture. Stir in oysters and next 2 ingredients. Spread in a lightly greased 13- x 9-inch pan; cover with aluminum foil.
Bake, covered, at 400° for 30 minutes. Uncover and bake 30 more minutes or until browned and thoroughly cooked. Let stand 10 minutes before serving.
Note: We tested with Weber New Orleans Cajun Seasoning and cornbread made with Martha White Cotton Country Cornbread Mix. (Do not use a sweet cornbread mix, such as Jiffy.)