In a large (1-gallon) zip lock bag, combine orange juice, olive oil, lime juice, lime zest, brown sugar, oregano, chili powder, salt and garlic. Add chicken, seal bag and marinate in refrigerator. For maximum flavor, it's recommended that you marinate for 4+ hours.
When ready to cook, heat a large grill pan on medium-high heat. Drizzle ~1 tablespoon olive oil in pan, add bell peppers and onion slices and cook until browned, about 6 minutes. Add a splash or two of reduced-sodium soy sauce during cooking. You want a nice color, but want the bell peppers to still have a bite to them. Remove from pan and keep warm.
Wipe grill with paper towel, add another drizzle of oil and carefully add chicken pieces (not the marinade) in a single layer. As chicken cooks, add two splashes of reduced-sodium soy sauce. Cook chicken 7-8 minutes or until browned and cooked through, turning once. Add onions and bell peppers back to the pan. Serve on warm flour tortillas with all the fixings.