Whisk oil, orange juice, vinegar, salt and pepper in a large bowl until well combined. Add watercress, watermelon and red onion; toss to coat. Divide among 5 plates. Top with goat cheese and hazelnuts to serve.
Tips: Melon selection & storage: Look for symmetrical unblemished melons, without flat sides, that have a creamy yellow spot on the bottom indicating ripeness.
At 92% water, this fruit should feel heavy when you heft it. Precut melon flesh should be dense, firm and appear moist. Store in the refrigerator for up to a week or keep in a cool, dark spot. Cover the cut surface of melon with plastic wrap and refrigerate.
To toast chopped hazelnuts: Toast in a small dry skillet over medium-low heat, stirring constantly, until fragrant and lightly browned, 2 to 4 minutes.