Crush garlic into small pieces in a mortar and pestle. Add olive oil, salt, oregano, and black pepper; mash into a paste.

Make 12 deep incisions a few inches apart in the roast using a sharp knife. Pack the seasoned garlic paste evenly into the incisions and over the roast.

Transfer roast to a large bowl. Pour in red wine, covering roast completely. Cover with plastic wrap and let marinate in the refrigerator, 24 hours.

Pour out red wine. Place roast fat-side up in a roasting pan; cover with aluminum foil. Let sit at room temperature, about 30 minutes.

Preheat oven to 400 degrees F (200 degrees C).

Place roast in the preheated oven; reduce temperature to 300 degrees F (150 degrees C). Bake until slightly pink in the center, about 4 hours.

Increase temperature to 400 degrees F (200 degrees C). Uncover roast and continue baking until fat is crispy and an instant-read thermometer inserted into the center reads 160 degrees F (71 degrees C), about 1 hour more.

Remove from oven and let rest before carving, 20 to 30 minutes.