Whisk orange juice, Dijon mustard, olive oil, Greek oregano, salt, and pepper together in a large bowl. Add potatoes and coat thoroughly with the mixture. Transfer potatoes to a large baking dish.
Stuff garlic cloves into the chicken underneath the skin. Place the chicken into the bowl with the remaining orange juice mixture; coat thoroughly and transfer to the baking dish on top of the potatoes. Pour any remaining orange juice mixture over the chicken and potatoes.
Bake uncovered in the preheated oven until no longer pink at the bone and the juices run clear, 60 to 90 minutes. An instant-read thermometer inserted into the thickest part of the thigh, near the bone, should read 165 degrees F (74 degrees C). Check every 30 minutes while roasting and add a bit of hot water if the potatoes seem too dry.
Remove chicken from the oven, cover with a doubled sheet of aluminum foil, and allow to rest in a warm area for 10 minutes before slicing.