Place potatoes in a large pot, and cover with salted water by about an inch. Bring water to a boil, and simmer just until fork tender, 15 to 20 minutes, depending on potato size.
Drain potatoes, cool just slightly, cut in half, and transfer to a large bowl. Add celery, olives, red onions, dill, and pimento, and stir until incorporated.
Whisk olive oil, lemon juice, oregano, salt, and pepper together in a small bowl, and drizzle over the warm potato salad. Sprinkle feta on top, and gently toss. Serve warm, at room temperature, or chilled.
