Heat olive oil in a large skillet over medium-high heat. Add the sliced chicken and cook for about 5-7 minutes, stirring occasionally, until the chicken is browned and cooked through.
Add the sliced bell pepper and onion to the skillet. Cook for an additional 3-4 minutes until the vegetables are tender.
Sprinkle the chili powder, cumin, garlic powder, onion powder, salt, and pepper over the chicken and vegetables. Stir well to combine and cook for another 1-2 minutes to heat through.
While the chicken and vegetables are cooking, warm the flour tortillas in a separate skillet or microwave.
To serve, place a portion of the chicken and vegetable mixture on each tortilla. Top with shredded cheese, salsa, and avocado slices if desired. Garnish with fresh cilantro.
Roll up the tortillas and enjoy your delicious chicken fajitas!