Preheat the oven to 350 degrees Fahrenheit.
In a large skillet over medium heat, heat up the olive oil and add in the chicken and onions. Cook until the chicken is cooked through and the onions are translucent.
In a small bowl, mix together the chili powder, garlic powder, salt and pepper. Add this mixture to the skillet and cook for another minute.
Warm up the tortillas in a dry skillet over medium heat. As they start to get soft, put them in a large bowl filled with warm water and let them sit for about 5 minutes so they can soften up some more.
Assemble your fajitas by distributing equal amounts of chicken mixture among each tort.