in a medium bowl, stir together the orange juice, soy sauce, olive oil, red onion slices, rosemary and garlic.
place the pork roast in a baking bag, and set in a roasting pan or baking dish. pour the orange juice mixture over the roast, making sure to coat entirely. close the bag according to package instructions.
bake for 2 to 2-1/2 hours in the preheated oven, until the internal temperature of the loin is at least 145 degrees.
remove from the oven, and let stand for about 10 minutes to settle the juices. carefully open the bag and remove the roast. slice and serve with a little of the drippings drizzled over.