Remove the ham from the packaging, including the plastic disk covering the bone. Place the ham in an oven bag, then twist and tie the bag shut. Place the ham cut-side down in a 9-inch by 13-inch baking dish. Allow the ham to sit at room temperature for 90 minutes.
Arrange an oven rack in the lower-middle position and preheat the oven to 250 degrees. Bake the ham until the center registers 100 degrees, about 1 1/2 to 2 1/2 hours (approximately 15 minutes per pound).
Meanwhile, in a medium sauce pan, bring the sugar, Dr. Pepper soda, orange juice, and mustard to a simmer. Cook until the mixture is syrupy and reduced to about 3/4 cup, about 8 minutes.
Remove the ham from the oven and roll back the sides of the oven bag to expose the ham. Brush the ham with about half of the glaze. Return the ham to the oven and bake 10 minutes longer, or until the glaze is sticky.
Remove ham from oven and remove the oven bag. Brush the ham with the remaining glaze and tent loosely with foil. Allow the ham to rest for 30 to 40 minutes.
Plastic oven bag: Look for "oven bags" by Reynolds and several other kitchen brands sold online and at many supermarkets. These are oven-safe and help keep the ham meat from drying out as it roasts.
Ham: For maximum flavor, seek out a bone-in spiral sliced ham. The bone imparts more flavor and juiciness, and the slicing style allows the glaze to permeate into the meat (not just coat the exterior).
Dr. Pepper: Regular soda, not diet.
Yield: My Dr. Pepper Ham recipe makes 20 entree-sized servings, ideal to feed a crowd plus enjoy for a couple rounds of leftovers.
Storage: Store leftovers covered in the refrigerator for up to 4 days.
Make ahead: The glaze can be made a day ahead and stored in the refrigerator. Reheat it on the stove before using.
Freezer: Slice and pack leftover ham into freezer-safe containers, then label, date, and freeze for up to 3 months. Thaw overnight in the refrigerator.