Place the pork, fat-side up, in a roasting pan fitted with a rack insert, and using a sharp knife, score the surface of the meat with small slits. Mash the garlic, oregano, salt, and pepper into a paste on a cutting board with the flat side of a knife; place the adobo in a bowl and stir in the oil and vinegar. Rub the garlic paste all over the pork, being sure to get into the incisions so the salt can penetrate the meat and pull out the moisture - this will help form a crust on the outside when cooked. Cover the pork with plastic wrap and marinate in the refrigerator for at least 3 hours or up to overnight.
Allow the meat to sit at room temperature for 30 minutes before cooking. Preheat the oven to 350 degrees F.
Roast the pork for 3 hours, uncovered, until the skin is crispy-brown. Let the meat rest on a cutting board for 10 minutes before slicing.