1. Preheat grill to medium (300°F to 350°F). For the dressing, in a small bowl combine ⅓ cup of the oil, vinegar, ½ teaspoon of the Greek seasoning, ¾ teaspoon of the oregano, and salt; set aside.
2. Pat fish dry with paper towels. Place fillets in a grill basket or on a sheet of heavy-duty foil; sprinkle evenly with remaining ¼ teaspoon Greek seasoning and oregano.
3. Place fillets on grill. Grill, covered, 4 to 6 minutes per side or until fish flakes easily with a fork. Remove from grill.
4. Place arugula on 4 plates. Top with oranges, olives and onion. Top each with 1 fillet, drizzle with dressing, and serve.
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