Preheat oven to 450 degrees. Place beets on a large piece of foil on a baking sheet. Drizzle with 1 tablespoon oil and season with salt and pepper. Fold foil around beets and crimp ends to form a packet. Roast until tender when pierced with a knife, 25 to 30 minutes. Remove beets from foil and let cool slightly.
Meanwhile, in a large bowl, combine 2 teaspoons oil with vinegar and season with salt and pepper. Add arugula, oranges, and beets and toss to combine. Top with pistachios and serve.