Wash and dry pork loin. Rub both sides with salt and pepper. Rub pork with olive oil. Sprinkle both sides with oregano, cumin, and orange zest. Place the pork in a slow cooker and top with onion and garlic. Squeeze orange halves over pork. Cook on LOW for 8 to 10 hours or on HIGH for 4 hours.
Once the meat is tender, remove from slow cooker and let cool slightly before pulling apart with a fork.
In a medium bowl, combine tomatoes, onion, cilantro, lime juice, salt and pepper.
Heat tortillas according to package directions. Add pork, salsa, shredded cabbage, and sliced radish.