Heat oven to 375° F. Tear off 8 squares of parchment paper. Arrange 4 squares on a flat surface.
Dividing evenly, place the carrots and leeks in the center of each square.
Place the halibut on the vegetables. Season with ½ teaspoon salt and ¼ teaspoon pepper. Drizzle with the oil and sprinkle with the oregano.
Top the fish and vegetables with the remaining squares of parchment. Fold over the edges several times to seal.
Transfer the packets to a baking sheet and bake for 15 minutes.
Using a paring knife, cut open the packets before serving. (Be careful of the escaping steam.)