Under running water, trim the outer layer of fat from the lamb and blot dry. Allow the lamb to reach room temperature and squeeze the juice from the lemons over the lamb. Slice cloves of garlic, lengthwise.
Put salt and pepper into a small container. Cut 6 small slots into each side of the leg of lamb, about 1-inch deep. Insert garlic slivers, followed by salt and pepper, into the lamb. Lightly sprinkle top of lamb with oregano. Pour the water into a roasting pan, add lamb, and cover with foil. Roast on lowest rack of oven for 40 minutes to 1 hour.
Remove foil, lower heat to 375 degrees and roast for 40 to 50 minutes, or until lamb reaches desired temperature. Remove and let lamb stand about 30 minutes as the lamb continues to cook. To test the temperature of the meat, insert a meat thermometer into the thickest part of the leg without touching the bone. For rare lamb, the thermometer should register 140 degrees. For medium, the thermometer should register 145 degrees.