Heat the oven to 425°F.
Toss the potatoes, onion, and garlic with 2 tablespoons of the oil and half the salt on a rimmed baking sheet.
Rub the chicken with the remaining oil, oregano, and salt. Nestle it skin side up among the vegetables and tuck in the lemon slices.
Roast until the skin is crisp and the chicken reaches 175°F, 40 to 45 minutes.
Squeeze the roasted lemon over everything and scatter with parsley.
Boneless thighs work too; start checking them at 25 minutes.