Place pork or chicken in the slow cooker.
Add orange juice, lime juice, zest, garlic, cumin, oregano, salt, pepper, and olive oil.
Cook on LOW for 8 hours or HIGH for 4–5 hours until very tender.
Remove meat, shred with two forks, then return to the cooking juices. Rest 10–15 minutes.
Optional: Spread shredded meat on a sheet pan and broil 3–5 minutes until lightly crispy.
Toss slaw ingredients together if using.
Assemble tacos with warm tortillas, mojo meat, slaw, avocado, cilantro, and lime.