If using leftover mashed potatoes, skip this step. In pot, cover diced potatoes with water by 1 inch. Sprinkle with salt, around 1/4 teaspoon, and bring to boil for 12 minutes. Drain potatoes, transfer back to pot and mash until smooth, creamy and free of lumps. Mix in 1 tablespoon oat milk and a sprinkle of salt and pepper.
In stand mixer, combine yeast, 2 tablespoons oat milk and 1/4 tsp. sugar. Set aside for 10 minutes until bubbles form and mixture is frothy. Prepare a 9x5-in. loaf pan with parchment paper or lightly grease to prevent sticking.