Put the olive oil, tomatoes, onion, mozzarella, basil, and salt in a bowl and mix together. Let stand at room temperature an hour or so.
Cook the pasta and drain, then stir into the tomato mixture.
Variation: Use cheddar cheese instead of mozzarella. Add some chopped parsley and crushed red pepper. Omit the pasta, cut down on the cheese, and serve it as a tomato salad on the side!