Boil potatoes in salted water. Cook until tender (about 15 minutes). Drain and cool. Cut potoates into desired size, and refrigerate until cold.
In a small bowl, stir together the ranch dressing mix, mayonnaise, and green onion. Cover, and refrigerate for about 2 hours to blend flavors. Remove from refrigerator and add milk to get desired consistancy.
Stir the mayonnaise mixture into the bowl of potatoes. Toss in cheddar cheese and crumble bacon into the bowl, and stir to distribute. Serve.