Preheat the oven to 300 degrees F.
In a small bowl, mix together the garlic, oregano, salt, smoked paprika, black pepper and the zest from both oranges. Rub the spice mixture all over, and well into, the pork roast.
Add the sliced onions to a large Dutch oven. Place the rubbed pork on top of the onions. Pour the chicken stock and orange juice over the pork. Cover, and bring to a simmer over medium-high heat, then transfer to the oven.
Cook for 4 hours--the pork will shred easily with a fork when it is done. Remove the meat from the pot and transfer to a baking dish. Let cool slightly, then shred using two forks.
Serve the pulled pork with soft rolls and any cooking liquid, if desired.