Using a mandoline, slice the potatoes crosswise into very thin rounds, about 1/16-inch thick. Place the slices in a large bowl and cover with cold water.
Rinse, moving the potatoes around in the water, then drain. Repeat as needed until the water runs clear rather than cloudy — this removes excess starch so the chips can crisp properly.
Spread the potato slices onto paper towels or a clean kitchen towel and pat completely dry. Dry out the bowl as well.
Return the dried potato slices to the dry bowl and add the oil, salt, and black pepper if using. Toss to coat evenly.
Air fry at 325°F for about 20 minutes, until golden brown and crisp, working in batches if needed to avoid overcrowding the basket.
Serve immediately for the best crunch.