Preheat oven to 350 degrees F. Thaw fish, if frozen. Rinse fish and pat dry with paper towels; set aside. In a medium bowl, toss sweet potato slices with oil. In a small bowl, combine oregano, chile pepper, cumin, salt, and black pepper. Sprinkle 1 teaspoon of the oregano mixture evenly on the salmon. Add remaining oregano mixture to sweet potatoes and toss to coat.
Tear off four 18-inch squares of heavy foil. Lightly coat one side of each foil square with nonstick cooking spray. In the center of each foil square, arrange one-fourth of the sweet potato slices, overlapping slightly. Top each sweet potato mound with a salmon fillet and arrange orange pieces atop salmon.
For each packet, bring up two opposite edges of a foil square and seal with a double fold. Fold ends to completely enclose fish and vegetables, leaving space for steam to build. Place the packets, seam sides up, on a very large baking sheet.
Bake for 30 to 35 minutes or until fish flakes easily when tested with a fork and sweet potatoes are tender. Open the packets carefully when checking doneness. Transfer contents of each packet to a dinner plate.