Heat oil in a medium Dutch oven over medium heat. Add onion and celery, and cook until soft and translucent, 10 minutes. Add garlic, and cook until soft, 2 minutes. Add chicken, orange juice, vinegar, currants, sugar, and peperoncini. Bring to a simmer; cover, and cook until chicken is cooked through, 20 minutes. Cut each piece in half.
Meanwhile, bring a pot of water to a boil. Add couscous, and cook until al dente. Drain. Serve chicken with sauce over couscous.