Place ham, cut side down, on a rack in large roasting pan. Pour 2 cups of water into roasting pan. Cover ham with aluminum foil.
Heat at 375 for 1 hour 30 minutes.
While ham is heating, whisk together brown sugar, orange juice, marmalade and mustard. Set aside.
Remove foil and brush ham liberally with about 3/4 cup of the orange glaze. Heat uncovered for 30 minutes or until ham is nicely colored and internal temperature registers 140 on an instant read thermometer.
Let ham stand 10 minutes before slicing. To slice, cut around center bone with a knife, freeing as many slices as you wish. Cut along the natural seams of the ham to remove slices separated from the bone. Serve with remaining glaze on the side. Garnish with orange slices, if desired.