Whisk 3 tablespoons oil, 3 tablespoons orange juice, 1 tablespoon vinegar, ¼ teaspoon salt and pepper in a large bowl until well-combined.
Add 8 cups watercress, 4 cups watermelon and ¼ cup red onion; toss to coat.
Divide among 5 plates.
Top with 2 ounces goat cheese and ½ cup hazelnuts to serve.
