Make sure the lamb is cleaned well and any hard fat is trimmed off. Make 8 slits in the leg and stuff them with the rosemary sprigs and garlic cloves. Rub the leg with the olive oil, dry oregano, lemon, salt, and pepper. Roast for about 30 minutes.
Lower the heat to 375 degrees F. Add water to the pan, if needed. Cook for about 3 1/2 hours. Remove from the pan and let rest. Skim the fat from the pan juices and pour into a bowl with the fresh oregano.