Preheat the oven to 400°F
Spread the fries in a single layer on a baking sheet
Lightly oil the fries if desired
Heat for 5 to 10 minutes
Flip halfway through
Remove when hot and crisp
Use an air fryer at 350°F for 3 to 5 minutes
Shake the basket halfway through
Reheat in a skillet over medium heat
Stir occasionally until warmed through
Avoid microwaving if you want them crispy