In a large bowl, whisk together the orange juice, mustard, olive oil, oregano, salt and pepper. Add potatoes and toss to coat. Place potatoes in a roasting tin.
Stuff the garlic cloves into the chicken under the skin. Place the chicken in the bowl with the remaining orange juice mixture and toss to coat. Place in the roasting tin on top of the potatoes. Drizzle over any of the remaining orange juice mixture.
Bake, uncovered, until potatoes are done and chicken's juices run clear, 60-90 minutes. Check every 30 minutes and add a little bit of hot water to the tin if the potatoes are drying out.