In a large pot, combine the mirin, soy sauce, ginger, chiles, orange slices, sesame oil and water. Add the pork roast, cover and refrigerate overnight.
Preheat the oven to 350°. Drain the pork and let it come to room temperature. Pat dry. In a medium, flameproof roasting pan, heat the vegetable oil. Add the pork roast and cook over moderate heat, turning occasionally, until browned all over, about 10 minutes. Transfer the roast to the oven, meaty side up, and roast for about 1 hour and 10 minutes, or until an instant-read thermometer inserted in the thickest part of the meat registers 135°. Cover the pork very loosely with foil and let rest for 15 minutes.
Using a long, sharp knife and using the bones as your guide, carefully carve the pork roast off the bones in one piece. Slice it thinly before serving. Reserve the rib rack for another meal or cut into individual ribs and serve them alongside the roast.