Heat oil in a large skillet over medium-high heat. Add garlic, onion, and sausage; cook and stir often until onion starts to brown, approximately 4 to 6 minutes. Add tomato sauce, pasta, olives, and spinach. Cook and stir often until bubbling and the spinach is wilted, approximately 3 to 5 minutes. Add 1 to 2 tablespoons water to keep the pasta from sticking. Stir in feta cheese and fresh basil. Serve.