Heat a large skillet over medium heat with a little bit of oil. Saute the onions until translucent, and then add the ground beef and saute until brown. Drain the excess fat, and then transfer the beef and onions to a large slow cooker. Add the beans, corn, tomatoes, green chiles, black olives, taco seasoning, and ranch dressing mix, as well as salt and pepper to taste, and stir all together.