Preheat oven to 500 degrees. In a small bowl, stir salt, pepper, oregano, garlic and 1/4 cup olive oil into a paste. Rub the mixture over both chickens. Distribute onion, celery, carrots and potatoes in the bottom of a large shallow roasting pan. Place chickens on top of the vegetables, breast side up. Stir together the lemon juice, chicken broth and remaining 1/4 cup olive oil; pour over chicken and vegetables. Bake, uncovered for 20 minutes. Turn the chickens over and roast, back side up for 20 more minutes, basting the vegetables every 10 minutes. Reduce heat to 450 degrees and turn the chickens breast side up. Cook for an additional 20 minutes, basting vegetables and chicken twice. The chicken is done when the drumsticks wiggle. easily.