Preheat the oven to 425°F and line a large baking sheet with parchment paper. Mix all the ranch seasoning ingredients in a small bowl and set aside 1 teaspoon for the drizzle. This homemade blend matters, since most store bought ranch packets contain buttermilk powder.
Toss the potatoes with the olive oil and the remaining ranch seasoning until well coated. Spread them on the baking sheet cut side down.
Roast for 30-35 minutes, tossing halfway through, until golden on the outside and tender inside.
Push the potatoes together into a tight pile in the center of the sheet. Sprinkle with the dairy free cheddar and bacon and bake for 5-7 minutes more until the cheese is melted.
Whisk the sour cream, mayonnaise, oat milk, lemon juice, and reserved ranch seasoning until smooth. Drizzle over the hot potatoes, scatter with green onions, and serve.