1. Slice orange crosswise into 1/4-inch-thick rounds; discard seeds. Line a shallow 2 1/2- to 3-quart casserole with orange slices.
2. Pull skin and fat off chicken and discard. Rinse chicken; lay in casserole.
3. Mix mirin, vinegar, cornstarch, oyster sauce, tamari, and ginger. Pour evenly over chicken.
4. Bake, uncovered, in a 375° oven, basting occasionally with pan juices, until chicken is no longer pink at bone (cut to test), 50 to 60 minutes.
5. Sprinkle with green onion.