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