Preheat oven to 400 degrees. Prepare macaroni and cheese in large saucepan as directed on package. Meanwhile, brown meat in a large nonstick skillet on medium-high heat; drain. Add tomatoes and oregano; mix well. Bring to a boil.
Add meat mixture to macaroni and cheese; spoon into 9-inch square baking dish. Sprinkle with cheese.
Bake for 10 minutes or until cheese is melted and the casserole is heated through.