Preheat the oven to 180°C (160° fan).
Thinly slice the potatoes lengthwise on a mandoline; pat slices dry with kitchen paper.
Heat the olive oil in a saucepan set over medium heat. Add the garlic and cook for 1 minute, swirling a few times.
Remove from the heat and stir in about half the oregano.
Working by the slice, brush the sweet potato with the oil, stacking them on their sides in a rectangular baking dish; pour any remaining oil over the top and season with salt and pepper.
Bake until golden brown and crisp, about 1 hour.
When ready, remove from the oven and let stand for 5 minutes.
Stir together 1 tbs flaked sea salt with the remaining oregano; sprinkle all over the potatoes before serving.