In the meantime, place a skillet over medium heat and add the oil to it. Cook the onion in it till it turns tender. Then, add in the garlic and cook it for about 2 minutes. Now, add the tomatoes as well, and cook the same till they turn slightly warm. With the help of a large spoon, remove the strands of the spaghetti squash. Mix the squash with the cooked vegetables, olives, cheese, and basil. Serve hot.