Remove the fell (tissue-like covering) from lamb with a sharp knife. Make small slits on outside of lamb; stuff with garlic. Rub surface of lamb with salt, pepper, and oregano. Wrap lamb in aluminum foil.
Place lamb on grill over medium coals. Insert meat thermometer, if desired. Cover grill, and open vent. Grill until desired degree of doneness: about 2 hours or 140° (rare); about 2 1/2 hours or 160° (medium); about 3 hours or 170° (well done).
Transfer lamb to a warm serving platter. Let lamb stand 10 minutes before slicing.