Whisk first 6 ingredients in small bowl. Gradually whisk in oil. Season with salt and pepper. Do ahead Can be made 1 day ahead. Cover; chill. Bring to room temperature and rewhisk before using. Place onion in large bowl. Add 1/3 of dressing; toss. Let marinate 20 minutes. Cut off peel and pith from oranges. Cut each orange crosswise into 8 slices. Add arugula, mint, and olives to bowl with onion; sprinkle with salt and pepper and toss. Add remaining dressing; toss. Divide salad among 6 plates. Tuck orange slices and ricotta salata slices into salads.