Make the orange marinade by whisking together the orange juice, soy sauce, rosemary, and garlic. Pour over pork tenderloin and marinate for at least one hour, preferably overnight.
Preheat oven to 400 degrees F (205 degrees C). Drain pork, reserving the marinade, and place on a baking sheet. Season with salt and pepper to taste. Roast for about 20 minutes until internal temperature has reached 145 degrees F (63 degrees C).
Meanwhile, strain the reserved marinade and bring it to a simmer in a small saucepan. Serve this as a sauce for the meat.