Rinse and tear spinach. Cover and store in refrigerator. For dressing, in a screw top jar combine oil, red wine vinegar, orange juice, mustard, and poppy seeds. Cover and shake well. Chill up to 24 hours.
When ready to serve, in a large salad bowl combine spinach, orange sections, chicken, and strawberries or cantaloupe. Shake dressing and drizzle over salad; toss to coat. Makes 4 to 6 main-dish servings.