Preheat oven to 425 degrees. Place beets on a large piece of foil on a baking sheet. Top with 2 teaspoons oil and season with salt and pepper. Fold foil around beets and crimp ends to form a packet. Roast beets on sheet until tender when pierced with a knife, 45 minutes to 1 hour. Remove beets from foil and let cool, then peel and cut into 1/4-inch-thick slices.
In a small bowl, whisk together remaining 3 teaspoons oil, shallot, and vinegar; season with salt and pepper. On a large platter, arrange beets and tomatoes, and season with salt and pepper. Drizzle with dressing and sprinkle with fresh oregano leaves.
This salad can be made ahead and enjoyed at room temperature. Add the oregano just before serving.