Add potatoes to a large pot of water and bring to a boil. Cook until the potatoes are soft, about 20 minutes.
Meanwhile, place the bacon slices in a large skillet over medium heat, and cook until crisp. Remove to paper towels.
Wipe out the skillet, return to stove, and pour in vegetable oil. Stir in mushrooms and onion, and cook until onion is soft and translucent.
When the potatoes are done, drain water, and place potatoes in a large bowl. Mash in butter and ranch dressing. With a potato masher or electric mixer, continue to mash until the potatoes are smooth. Stir in the mushrooms, onions, and crumbled bacon.
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