Cook the orzo according to the instructions on the packet, then rinse it with cold water to cool it. Chop the cherry tomatoes in half. Dice the bell pepper and cucumber and crumble the feta cheese. Clean the onions, cut them in half, cut them in half again and cut into slices. Stir the red wine vinegar into the mustard and garlic and add the oil bit by bit, stirring all the time. Season with salt and pepper. Chop the oregano and 1/3 of it to the dressing. Mix half the feta cheese, half the tapenade and all the other ingredients with the dressing. Garnish the salad with the remaining feta cheese and tapenade and sprinkle with the remaining oregano.