Rinse and pat dry the pork roast. Coat with the salt and pepper and press it down into the meat. Mix together the oregano, cumin, and olive oil and spread over the roast. Set the roast inside a slow cooker, with the fat side up.
Toss in the onion, Serrano pepper, and garlic, then squeeze the orange quarters of their juice over the roast and veggies. Toss the rinds into the slow cooker, then cover and cook on low for 8-10 hours, or on high for 4-6 hours. Once cooked through and tender, remove from the slow cooker and gently shred with forks.
Heat the canola oil in a large skillet over medium, medium-high heat. Add the shredded carnitas and let cook for a few minutes, until the bottom is browned and crusty. Remove from heat and serve in corn tortillas with onion, cilantro, and fresh chile.