Heat oven to 350°. In a food processor, add 2 ½ cups oats and process into a very fine meal. Pulse in flax, baking powder, cinnamon, and salt. Add oil, mashed potato, syrup, sugar, and vanilla and process until combined. Transfer to a bowl; stir in remaining oats. In batches, drop 2-tablespoonful mounds onto parchment-lined baking sheets. Flatten each with the palm of your hand; they will not spread while baking. Bake until golden brown at edges, about 15 minutes. Let cool 15 minutes on baking sheet before transferring to a rack to cool completely.