Heat the olive oil in a large skillet over medium-high heat.
Add the diced onion and ground turkey to the skillet and cook about 5 minutes, breaking the meat apart with a spoon, until the turkey is browned and the onion is softened.
Stir in the minced garlic and cook for 30 seconds until fragrant.
Transfer the browned turkey mixture to the slow cooker and add the tomato sauce, ground cumin, chili powder, dried oregano, smoked paprika, cayenne pepper (if using), and salt; stir to combine.
Cover and cook on high for 2–3 hours until flavors meld; stir once or twice during cooking if possible.
Warm the corn tortillas, spoon the turkey mixture into tortillas, and top with shredded lettuce, diced tomato, avocado, or shredded cheese as desired before serving.