Pound the garlic cloves and salt to a paste using a mortar and pestle. Add the peppercorns and oregano, pounding well after each one to incorporate them into the paste. Stir in the oil and vinegar.
To make the pork:
Up to 3 days before you serve the roast, set it in a bowl, skin side up. With a paring or boning knife, make several slits about ½ inches apart through the skin of the roast and into the meat. Make the slits as deep as you can. Wiggle a finger in the slits to open them up a bit and then fill each one with wet rub using a teaspoon. (A pair of latex gloves comes in handy when it comes time to rub the wet rub into the pork.) Do the same on all sides. If you have rub left over, smear it all over the outside of the roast. Refrigerate, covered, at least 1 day or up to 3 days.
Preheat the oven to 450°F.
Set the roast, skin side up, on a rack in a roasting pan. Roast for 1 hour, turn the heat down to 400°F, and roast until the skin is a deep golden brown and crackly and with no trace of pink near the bone, about 1 ½ hours or until an instant reading thermometer inserted near the bone registers 160°F. Let the roast rest at least 15 minutes before carving.
To serve, remove the crispy skin. It will pull right off in big pieces. Cut them into smaller pieces—kitchen shears work well for this—and pile them in the center of the platter. Carve the meat parallel to the bones all the way down to the bone. (It will get trickier to carve neat slices as you get near the bone; don’t let that bother you.)
The salt keeps the garlic from flying all over the place as you pound them together. (This wet rub will keep for 5 to 6 days in the refrigerator, which gives you a chance to try it on anything you like, from fish fillets and pork chops to turkey cutlets and steaks.)