Heat oven to 450 degrees F. On a large rimmed baking sheet, toss carrots with oil, paprika, nutmeg, 1/2 teaspoon salt, and 1/4 teaspoon pepper, then toss with oregano. Transfer half to second sheet and roast until golden brown and tender, 20 to 25 minutes. Gently toss with butter and vinegar and sprinkle with pistachios.