Let the ham sit at room temperature 30 minutes. Preheat the oven to 325 degrees F. Trim off any skin from the ham if needed. Score the ham through the fat in a diagonal crosshatch pattern without cutting into the meat. Place the ham, flat-side down, on a rack in a roasting pan. Pour 1/4 inch of water into the bottom of the pan. Roast ham until it reaches an internal temperature of about 130 degrees F, about 2 1/2 hours or 15 minutes per pound.
Meanwhile, combine the sugar, paprika, garlic powder and oregano in a small bowl and stir until well mixed. Whisk in the olive oil until the mixture is a smooth paste.
Remove the ham from the oven when it has reached 130 degrees F internally. Increase the oven temperature to 425 degrees F. Spoon or brush the mustard all over the ham. Spread the spice paste with a spoon as evenly as possible all over the ham. Add water as needed to the bottom of the pan. Return the ham to the oven and roast until the paste forms a very light brick red crust, turning once halfway through, about 25 minutes more.