Heat oil in a large straight-sided skillet over medium-high heat. Add sausage, onion and garlic; cook, stirring often, until the onion is starting to brown, 4 to 6 minutes. Add tomato sauce, spinach, pasta and olives; cook, stirring often, until bubbling hot and the spinach is wilted, 3 to 5 minutes. Add 1 to 2 tablespoons water, if necessary, to keep the pasta from sticking. Stir in feta and basil, if using.