In salted boiling water, cook the potatoes until tender. Drain and allow to cool to room temperature. Peel and cut into 4 to 6 pieces. Transfer to a medium bowl. Add the red onions, tomatoes, olives and capers.
In another bowl, combine the vinegar, salt, pepper and oregano. Slowly whisk in the olive oil. Pour over salad mix. Taste and season with lemon juice.
Divide salad into 2 and mound onto salad plates. Surround with caper berries.