With a sharp knife or poultry shears, remove the backbone from the chicken. Turn it breast side up. With the heel of your hand, press down to break the breastbone, and open the chicken flat like a book. Insert a skewer through the chicken, at the thighs, to keep it flat during cooking.
Place the chicken in a shallow baking pan and pour on the lemon juice to coat. In a small bowl, combine the garlic, cayenne, paprika, oregano, pepper and oil. Mix well. Rub evenly over the surface of the chicken. Cover and let marinate at room temperature 2 to 3 hours, or chill overnight (return to room temperature before roasting).
Put the pan in a cold oven and set the temperature to 400 degrees. Roast until the chicken is done, about 1 hour. To test, prick with a skewer (the juices should be clear).