Place the cucumber, peppers, tomatoes and red onion in a large bowl.
For the vinaigrette, whisk together the garlic, oregano, mustard, vinegar, salt and pepper in a small bowl. Still whisking, slowly add the olive oil to make an emulsion. Pour the vinaigrette over the vegetables. Add the feta and olives and toss lightly. Set aside for 30 minutes to allow the flavors to blend. Serve at room temperature.