ADD potatoes to a large pot of cold salted water. Bring to a boil and cook until tender, 15 min. Drain and run under cold water for 30 seconds, until cool enough to handle but still warm. Halve or quarter larger potatoes and set aside.
DRESSING: Whisk oil, lemon juice, garlic, oregano and salt in a large bowl. Stir in sun-dried tomatoes, olives and feta. Set aside.
ADD potatoes and toss to coat. To serve warm, stir in greens. If serving cold, chill potato salad and stir in greens just before serving.