Mash the garlic, salt and peppercorns into a paste, using a mortar and pestle or a food processor. Stir in sour orange juice, onion and oregano. Let mixture sit for 30 minutes.
CAREFULLY whisk the garlic-orange juice mixture until well blended. Let cool before using as a marinade. Pierce the pork roast several times all over with a fork or sharp knife.
Place it in a roasting pan and pour garlic mixture (save a little for basting while roasting) over pork. Cover and let sit in the refrigerator for two or three hours.