Place chicken breasts or thighs in the bottom of the slow cooker.
Layer sliced bell peppers and onions over the chicken and sprinkle minced garlic on top.
In a small bowl, mix olive oil with chili powder, cumin, paprika, onion powder, garlic powder, oregano, salt, and black pepper.
Drizzle the seasoning mix over the chicken and vegetables, then toss gently to coat evenly.
Cover and cook on low for 6–7 hours or on high for 3–4 hours until chicken is fully cooked and vegetables are tender.
Remove chicken from the slow cooker and shred it with two forks.
Return shredded chicken to the slow cooker and stir to combine with the juices and peppers.
Warm the tortillas and fill them with the chicken and pepper mixture.
Top with desired fajita toppings such as cheese, sour cream, salsa, cilantro, and a squeeze of lime.