Mix oregano, chili powder and salt. Cut each fish fillet in half lengthwise. Place in shallow glass dish. Coat fish on both sides with spice mixture. Cover. Refrigerate up to 1 hour to marinate.
Meanwhile, mix lime juice, sour cream and chopped cilantro until well blended. Add fennel; toss to coat well. Refrigerate until ready to serve.
Preheat greased grill to medium. Grill fish 4 minutes on each side or until fish flakes easily with fork. Meanwhile, grill tortillas 1 minute or just until warmed, turning after 30 seconds. Wrap in foil or towel to keep warm.
To serve, top each tortilla with fennel slaw, fish, tomato slices and orange sections.