Using a food processor, grind the garlic, salt, peppercorns and oregano. Add the olive oil and vinegar and process into a paste.
Set the pork roast in a bowl. Using a sharp knife, make about a dozen 1 1/2-inch-long slits all over the roast, cutting as deeply into the meat as you can. Fill each slit with about 1 teaspoon of the garlic paste. Rub any leftover paste all over the roast. Cover and refrigerate for 1 to 3 days.
Preheat the oven to 450°. Set the roast, skin side up, on a rack in a roasting pan and roast for 1 hour. Lower the oven temperature to 400° and roast until the skin is golden brown and crackly and the meat registers 160° on an instant-read thermometer, about 1 1/2 hours; let rest for at least 15 minutes before carving.
To serve, remove the crispy skin (it will pull right off in nice, big slabs), cut into pieces and pile in the center of a platter. Carve the meat parallel to the bone and place the slices around the pieces of skin.