Remove giblets and neck from turkey. Pat dry and place, breast down, on a clean cutting board. Using sharp kitchen shears, cut down either side of the backbone and remove. Lightly score breastbone, then flip turkey over. Turn legs out to either side of the bird, then press down on the center of the turkey breast until it cracks and flattens. Tuck wing tips behind bird (NOTE: If you prefer, you can ask your butcher to spatchcock your turkey for you).
In a small bowl, stir together butter and marmalade until well combined. In a second small bowl, combine poultry seasoning, salt, and pepper. Carefully loosen the skin around the entire turkey, using your hand and being careful not to break the skin. Work half of the butter mixture under the skin of the bird. Rub the remaining butter mixture on the outside of the skin and season all over with the seasoning mixture.
Spread orange slices evenly over a half sheet tray and place turkey on top. Transfer to oven and immediately reduce temperature to 325 degrees.
Meanwhile, combine orange juice and jellied cranberry sauce in a small saucepan over medium heat and cook, whisking occasionally, until the jelly melts. Continue simmering until glaze thickens, about 15 minutes. Keep warm.
After turkey has roasted for 1 hour, brush on the glaze. Continue roasting until a meat thermometer inserted into the breast registers 150 degrees and the thigh registers at least 165 degrees, another 30-60 minutes. Remove from oven and transfer turkey to a grooved cutting board. Allow to rest at least 15 minutes before slicing and serving.