The night before cooking, use a small paring knife to poke 10 holes in your pork shoulder and stuff a garlic clove in each one. Rub the olive oil all over the pork then thoroughly massage the shoulder with oregano and sazon, and season liberally with salt and pepper. Place the pork in a roasting pan and refrigerate, uncovered, overnight.
Heat oven to 300 degrees. Remove the pork from the refrigerator while the oven heats.
Roast the pork shoulder, flipping the roast every hour so that it cooks evenly, until the internal temperature reaches 180 degrees, about 5 hours. Raise the oven temperature to 400 degrees. Arrange the pork shoulder skin side up and roast until the internal temperature reaches 195-200 degrees, about 1 ½ hours more.
Let pork roast rest for about 15 minutes, giving the skin a chance to crisp, before shredding and serving.