Cut the cucumbers in half lengthwise, and then slice them into 1 to 2 inch thick pieces and place them in a large salad bowl. Cut the tomatoes in half, then slice them into triangular chunks, roughly 2 to 3 inches wide, and place the chunks in the bowl. Add the kalamata olives and sliced onions to the bowl and mix well.
Sprinkle the olive oil, vinegar, salt, pepper, and oregano over the salad and stir until the fruits and vegetables are completely coated in the spices. Crumble the feta cheese over the salad and toss gently with salad tongs to distribute. Serve chilled.