In large bowl, whisk together the orange juice, mustard, olive oil, oregano, salt, and pepper. Stir the potatoes into the bowl to coat with orange juice mixture. Remove potatoes with a slotted spoon, and place them into a large roasting pan.
Cut slits into the lamb meat, and stuff the garlic cloves into the slits. Rub remaining orange juice mixture from bowl all over the lamb, and place the lamb on top of the potatoes in the roasting pan. If there's any remaining orange juice mixture, pour it over the lamb.
Roast in the preheated oven until the potatoes are tender and the lamb is cooked to medium, about 1 hour. A meat thermometer inserted into the thickest part of the meat should read 140 degrees F (60 degrees C). Check every 20 to 30 minutes while roasting, and add a bit of hot water if you find the potatoes are drying out. If the lamb finishes cooking before the potatoes, remove the lamb to a cutting board or serving platter and cover with foil while the potatoes continue to bake in the oven.
