Heat oven to 400° F. Cook the ravioli according to the package directions, drain, and return them to the pot.
Meanwhile, heat the oil in a large skillet over medium-high heat. Add the sausage and cook, tossing occasionally, until brown, 2 to 3 minutes.
Toss the ravioli with the marinara sauce and sausage and transfer to a shallow 2-quart baking dish.
Sprinkle with the mozzarella and bake until bubbling and beginning to brown, 15 to 20 minutes.