Combine garlic, oregano, cumin, 1 1/2 teaspoons salt and 3/4 teaspoons pepper. Toss with pork and place in a 6 quart pressure cooker. Add 1/4 cup water and secure lid. Bring to high pressure over medium-high heat; reduce heat and cook until meat is fork tender, about 40 minutes (adjust heat to maintain pressure). Remove from heat, vent pressure, and remove lid. Using two forks, shred pork and serve with tortillas and desired toppings, such as pickled red onions, sour cream, cilantro leaves, and lime wedges.