Dice the onion and peel the garlic cloves. Place the onion, garlic, and cinnamon sticks in the bottom of the slow cooker.
In a small bowl, combine the oregano, cumin, salt, pepper, and chipotle powder. Cut the pork roast into 2-inch cubes. Place the cubed meat in a bowl and sprinkle the spice mix over top. Toss the meat in the spices until evenly coated. Place the seasoned meat in the slow cooker on top of the onion, garlic, and cinnamon.
Use a vegetable peeler to remove the thin layer of orange zest from one of the oranges. Squeeze about 1/2 cup juice from the oranges. Pour the orange juice over the meat in the slow cooker, than sprinkle the pieces of orange zest over top.
Place a lid on the slow cooker, turn it on to high, and cook for 5 hours, or until the meat is tender and falls apart.
Use tongs to stir and shred the meat, and remove the cinnamon sticks. Either divide the meat and liquid into smaller containers to refrigerate for later, or transfer the meat and some of the liquid to a skillet (preferably non-stick or cast iron). Cook the meat over medium flame until browned and crispy on the edges. Avoid stirring the meat too much as it will break down into tiny pieces. Use the crispy meat as a filling for tacos, burritos, nachos, or burrito bowls.