Turn the slow-cooker on high, let it heat for 5 minutes, and add olive oil, onion, and garlic. Stir and cover. (The pot won't get hot enough to brown them.) Cook for 1 hour, or until onion is soft and wilted. Uncover and add tomatoes, parsley, basil, oregano, salt, and pepper. Reduce the heat to low, and stir. Cook with the cover off for approximately 4 hours. The sauce is done when it is thick and not watery.