In a deep, 10-inch skillet, heat the oil. Add the sausage and cook over high heat, pressing to flatten, until cooked through,
10 minutes. Transfer to a plate, leaving the fat in the pan. Break the sausage into bite-size pieces. Add the onion and garlic to the skillet. Cover and cook over moderate heat, stirring occasionally, until softened, 4 minutes. Add the orzo and cook, stirring, for 3 minutes. Add the stock and 2 cups of water and cook, stirring constantly, until the orzo is al dente and suspended in a thick creamy broth, about 15 minutes. Season with salt and pepper.