In a bowl, mix together the salt, black pepper, cumin, oregano, and cayenne pepper. Rub the spice mixture all over the pork shoulder pieces.

In a skillet, heat the vegetable oil over medium-high heat. Brown the pork pieces on all sides, then transfer to the slow cooker.

Add the chopped onion, minced garlic, orange juice, and lime juice to the slow cooker. Stir to combine.

Cover and cook on low for 6-8 hours or until the pork is tender and easily shredded.

Once cooked, remove the pork from the slow cooker and shred it with two forks. Return the shredded pork to the slow cooker and mix with the juices.

Serve the carnitas in tacos, burritos, or on their own with your favorite toppings.