Zest the orange and mince the garlic. Mix the garlic, juice, oil, orange zest, oregano, cumin, salt, and pepper together to make the flank steak marinade.
If your steak is not a uniform ½-inch thick (1.27 cm), pound it with a mallet to get it there. This will also help to tenderize the steak.
Pour the marinade over the flank steak. Allow it to marinate in a covered container in the fridge for 8-24 hours. (I usually just let it marinate overnight.)
Let the steak rest at room temperature for 30-60 minutes after marinating.
Preheat the air fryer to 375°F (190°C) for 5 minutes.
Air fry flank steak for a total of 8-10 minutes at 375°F (190°C). Flip the steak halfway through cooking time, and use the top rack position if you have an air fryer oven. (4 minutes per side got me medium rare, while 5 minutes per side got me medium/medium well done.)
Check the steak for doneness using a meat thermometer. (Use the handy temperature chart for cooking flank steak in the above article.)
After the steak is cooked, allow it to rest for 10 minutes. Slice the steak into thin slices diagonally against the grain and serve.