Remove the tough, fibre-like covering from the leg. Score the fat in a criss-cross pattern. Crush the garlic and mix it with the orégano, salt, pepper and olive oil. Rub into the meat, cover and refrigerate for 24 hours.
Before cooking let the meat come to room temperature. Drain off any liquid that has collected and pour it over the meat as a marinade. Put the lamb on a rack in a roast