Add the pork shoulder to the slow cooker. Using a small knife cut slits into the pork, about an inch down.
In a large bowl add the olive oil. Cut the lemon, lime and orange in half. Squeeze the juice out of the citrus into the bowl. Add the oregano, cumin, garlic, salt and pepper. Whisk until combined.
Pour the mixture over the pork roast. Using a spatula squish the garlic and sauce into the pork.
Add the onion rounds.
Cover and cook on LOW for 10 hours without opening the lid during the cooking time.
Shred the meat with 2 forks and remove the bone. I left about half of the juices with the meat to keep it moist.