Heat oven to 325°F. On rack in shallow roasting pan, place ham, fat side up. Insert ovenproof meat thermometer so tip is in thickest part of ham and does not touch bone or rest in fat. Cover loosely; bake 1 hour 30 minutes.
Remove ham from oven. Pour drippings from pan. Remove any skin from ham. In small bowl, mix brown sugar, vinegar and mustard. Pat or brush mixture on ham.
Bake uncovered 20 to 30 minutes more or until thermometer reads 135°F. Cover loosely; let stand 10 minutes or until thermometer reads 140°F. Garnish with orange slices.