Place the ham on the rack in the roasting pan and rub it down with the olive oil. Score the surface in a diamond pattern about ½ to ¾-inches (1.25 to 1.9 cm) deep. Place the peppercorns at the corners of the diamonds.
Insert the meat thermometer into the deepest part of the ham, not touching the bone. Cover the top of the ham with foil.
Roast in the preheated oven. Baste the ham with some of the glaze when the temperature reaches 120° F/49° C and place the foil back on top. Continue cooking the ham until the temperatures reaches 148° F/64° C, about 3-4 hours total. Remove it from the oven and allow it to rest, covered, for 10 minutes. If roasted with the skin on, remove it and the top layer of fat before slicing. Serve with remaining glaze on the side.
GLAZE: While the ham cooks, stir the maple syrup, brown sugar, whiskey, and mustard and cook in the small saucepan over medium heat until it starts to boil. Reduce heat to low and continue cooking until the glaze reduces by one-third. Remove from heat and set aside.