Trim any excess thick fat from the pork shoulder and cut it into 2-inch chunks.
In a small bowl, whisk together the dried oregano, ground cumin, garlic powder, onion powder, sea salt, and black pepper.
Rub the spice mixture evenly over all sides of the pork chunks.
Place the seasoned pork into the slow cooker and pour the orange juice and lime juice over the top.
Cover and cook on low for 8 hours or on high for 4 to 5 hours until the meat is fork-tender.
Once cooked, remove the pork from the slow cooker and shred it using two forks, discarding any large pieces of fat.
For a true carnitas finish, heat a non-stick skillet over medium-high heat and sear the shredded pork for 3-5 minutes until the edges become golden and crispy.
Warm the corn tortillas in a dry pan or over an open flame until pliable.
Divide the crispy pork between the two tortillas.
Top with diced red onion, fresh cilantro, and a slice of avocado before serving.