Heat the oven to 425°F and line an 18-by-13-inch rimmed metal sheet pan with foil. Mix 1 tablespoon oil, orange juice, cumin, ancho chile, oregano and 1/2 teaspoon salt in a bowl and coat the whole thighs.
Toss peppers and onion with remaining oil and salt on one side of the pan. Arrange chicken on the other side in a single layer without covering the vegetables.
Roast for 15 minutes, turn the vegetables and continue for 10 to 15 minutes until the chicken reaches 165°F and the pepper edges brown.
Transfer cooked chicken to a clean board and rest for 5 minutes. Wrap tortillas in foil and warm on a cleared corner of the pan during that rest, leaving the vegetables in the oven.
Slice the thighs across the grain, toss with the peppers and lime juice and divide among the 8 tortillas. Serve 2 fajitas per portion.