Place the potatoes and water in a pot, cover, and bring to a boil over medium-high heat. Reduce the heat to medium and cook until tender but firm, about 25 minutes. Drain, remove, and let cool.
Slice the potatoes into large chunks and place them in a large bowl wih the tomatoes, onions, olives, and capers. Toss together.
Drizzle with the olive oil and vinegar and add the oregano. Toss to combine well. Serve immediately. (This salad is good served at room temperature or chilled.)
