Place garlic, oregano, salt, and pepper in a food processor. Pulse until the garlic is broken down. Slowly add the olive oil and vinegar and process until a relatively smooth paste is formed. Scrape down often.
Using a very sharp knife, remove the fat cap from the roast leaving it attached on one end (the narrowest) so you can lift it like a “flap.” Stab the roast all over, deeply, forming pockets throughout the meat. Rub the paste mixture into the roast, making sure to get some of the mixture into the pockets. Place fat cap back on top.
Lay overlapping sheets of plastic wrap on the bottom of a shallow dish, using enough to hang off the sides and enough to wrap the shoulder in. Place the shoulder in the dish and pull the plastic wrap up and over, sealing tightly. Marinade for at least 6 and preferably 24 hours. If there is no time to marinade, it will still be good, but the longer, the better.
When ready to cook, remove roast from oven, unwrap and let sit for an hour at room temperature. Preheat oven to 400 degrees F. for at least 30 minutes.
Place pork in a pan at least 2 to 2 1/2 inches deep. Cook, uncovered, for one hour at 400 degrees F. Turn oven down to 325 degrees F. and continue to roast until roast is tender and when a fork inserted in the side will turn easily. Calculate around 20 minutes per pound.
Watch the drippings in the bottom of the pan and add a little water if they begin to look dry. When finished if the fat cap isn’t rendered to your liking, either remove it and place it on a rack over a small pan and turn the oven up to 400 degrees to crisp it, watching carefully, or leave the roast in the oven and turn the oven up to 400 degrees until the fat cap is crisp.
Let roast sit for 15 to 30 minutes at room temperature, lightly covered with foil, before serving. Slice or break apart the shoulder meat, break the cueritos (crisped fat) into pieces and arrange around the meat. Keep them dry. Any juices may be defatted and drizzled over the meat.
Note: If your pork shoulder does not have a fat cap, simply stab it all over, rub, marinade, bring to room temperature & roast the whole time at 325 degrees.