01 - Set the oven temperature to 400°F and allow it to fully heat.
02 - Combine olive oil, chopped oregano, minced garlic, lemon zest, lemon juice, salt, and black pepper in a small bowl, stirring until well blended.
03 - Pat the chicken dry with paper towels. Evenly rub the marinade over the entire chicken, including beneath the skin and inside the cavity.
04 - Fill the chicken cavity with half of the onion wedges and several lemon slices.
05 - Place remaining onion wedges and lemon slices in a roasting pan. Set the chicken breast-side up atop them.
06 - Roast for 1 hour and 10 minutes, basting occasionally with pan juices, until the skin is golden brown and the internal temperature in the thickest thigh registers 165°F.
07 - Cover the chicken loosely with foil and let it rest for 10 minutes to redistribute juices.
08 - Adorn with fresh oregano sprigs if desired, and serve accompanied by pan juices.