Heat oven to 450° F. Place the chicken in a large cast iron or other ovenproof skillet; tie the legs with kitchen twine and season with 1 teaspoon each salt and pepper. Roast for 30 minutes.
Add the garlic, oregano, and lemon to the skillet; drizzle with the oil. Continue to roast, tossing the garlic and lemon once, until a thermometer inserted into the thickest part of a thigh registers 165° F, 25 to 30 minutes more. Let rest 10 minutes before carving.
Discard the oregano sprigs. Serve the chicken with the garlic and lemon.