Heat the grill until hot. Finely chop the oregano and garlic together into a fine paste, setting 1 tablespoon aside. Season the chicken on the outside and under the skin with salt and pepper, then rub the remaining herb mixture under the skin of the chicken.
Place the chicken on the grill; cook, turning once, for 10 minutes.
Meanwhile, mix the marmalade, reserved herb paste, and vinegar until well blended. Brush onto the chicken a few times, turning it once or twice, for 10 to 15 minutes longer or until the chicken is golden brown and charred (the meat should no longer be pink near the bone and juices run clear; make sure it's 165 degrees in the interior).
Nutrition information per serving: