In large bowl whisk together the orange juice, mustard, olive oil, oregano, salt and pepper.
Once combined add potatoes to bowl and coat thoroughly with mixture. Transfer the potatoes to a large roasting tray.
Cut 2cm deep slits into the lamb and stuff the garlic cloves into the slits. Place the lamb into the bowl with the remaining orange juice mixture; coat thoroughly and transfer to roasting tin on top of potatoes. If any of the orange juice mixture remains in the bowl pour over the lamb and potatoes.
Bake uncovered until potatoes are done and lamb is medium / medium-well; approximately 60 minutes. Check every 20 to 30 minutes while baking and add a bit of hot water if you find the potatoes are drying out.