Combine chili powder, oregano, salt and pepper in a bowl. Pat pork dry with paper towels. Separate spice mixture in half, and rub half of spice mixture onto the pork. Heat oil in a large pan and brown the pork on all sides. Remove to a plate.
In the same skillet, add onions and and cook over medium heat, stirring occasionally, until softened and browned, about 8 minutes. Stir in flour and remaining spice mixture, and cook about 1 minute until fragrant. Add tomato sauce and chipotle, scraping up any browned bits, and cook until it starts thicken, about 1-2 minutes . Pour mixture into slow cooker.
Nestle pork into the liquid in the slow cooker. Cook on low for 7-8 hours, or on high 4-5 hours, until a fork slips easily in and out of the pork and it is pull apart tender.
Remove pork to a cutting board and let rest 15 minutes. Slice to serve. If desired, strain the sauce including solids into a fat separator and let sit until fat separates. Combine defatted sauce, solids and cilantro or parsley and spoon over pork before serving.
Enjoy!