Peel the bananas and slice them into thin, even rounds.
Toss the slices gently with oil to coat lightly.
Mix sugar and cinnamon in a small bowl and sprinkle it over the banana slices.
Place slices in a single layer in the air fryer basket.
Air fry at 160°C (320°F) for 12 minutes, flipping halfway through.
Remove and let cool on a rack — they crisp up more as they cool.