Set out a large salad bowl, and a small bowl for the vinaigrette. In the small bowl, whisk together the oil, vinegar, garlic, oregano, 1/2 teaspoon salt and 1/4 teaspoons ground black pepper. Set aside.
Cut the cucumber into quarters, lengthwise. Then slice it into 1/2 inch chunks. Cut the grape tomatoes in half. Seed and chop the bell pepper into 1/2 pieces. Cut the feta into 1/2 inch cubes. Chop the mint and slice the red onion.
Place all the fresh produce in the large bowl. Pour the vinaigrette over the top. Gently toss to coat all the chunks in dressing. Then cover and refrigerate until ready to serve.