Mix garlic, oil, vinegar, sazon, adobo, pepper, and oregano together to form a paste.
Cut a wide pocket at the large end of the roast to separate skin from the fat, leaving skin attached at the sides and stopping before roast narrows to bone. Make 1-inch deep slits under the skin and on all meaty sides, twisting the knife to widen the openings.
Stuff the garlic paste into the slits and rub all over the meat. Transfer to a large zip-top bag or ceramic or glass bowl, cover, and let marinate in the refrigerator for at least 2 hours or up to 8 hours.
Preheat the oven to 400 degrees F (200 degrees C).
Remove roast from the refrigerator. Place the roast in a pan on a rack, fat-side up. Cover with foil.
Roast in the preheated oven for 1 hour. Reduce the oven temperature to 300 degrees F (150 degrees C) and remove foil. Continue to roast until cooked through and an instant-read thermometer inserted into the center should read at least 175 degrees F (79 degrees C), about 4 hours more.