Using a mandoline or sharp knife, cut 2 medium russet potatoes lengthwise into 1/8-inch thick planks. Stack a few planks and cut lengthwise into 1/8-inch wide matchsticks. Place in a large bowl and cover by a few inches with cold water. Let soak 30 minutes.
Drain the potatoes, then pat dry with a clean kitchen towel to remove as much moisture as possible.
Heat 4 cups vegetable oil over medium-high until 350°F. Meanwhile, fit a wire rack over a rimmed baking sheet, or cover the baking sheet with paper towels.
Working with a handful of potatoes at a time, add to the pot and cook, stirring with a spider or slotted spoon a few times, until golden and crisp, 3 to 5 minutes.
Using the spider or slotted spoon, transfer the potatoes to the prepared baking sheet and season with kosher salt and Old Bay. Repeat frying the potatoes.