Preheat oven to 450 degrees. Pierce sausage all over with a fork; place on a rimmed baking sheet with potatoes and rosemary. Drizzle all with oil; season generously with salt and pepper. Toss to coat, and spread in an even layer.
Roast, tossing occasionally, until sausage is browned and potatoes are tender, 30 to 35 minutes. If desired, halve sausages crosswise before serving.