Wash the chicken and dry it with paper towel. Season with salt and pepper inside and outside of chicken. Also season with oregano inside of it and place lemon wedges and onion in it. Spread the butter all over the chicken with your hand. Place chicken in a oven dish, cover with aluminum foil. Preheat the oven 500 F (260 C), cook for about half an hour. Remove the aluminum foil, reduce the heat to 350 F (180 C), cook another 1 hour. Let sit chicken 10 minutes, then cut in four.