In a small lidded jar, combine the olive oil, vinegar, lemon juice, oregano, salt, and pepper. Shake well to combine.
Make the salad
Combine the cucumber, pepper, tomato, feta, onion, and olives in a large salad bowl. Pour the dressing over and mix to combine.
Cover and chill until you're ready to serve.
Notes
If you prefer a more tangy dressing, add an extra 1 to 2 tablespoons vinegar to the dressing mixture.
The oil in the dressing may solidify after sitting in the fridge. If this happens, let the salad sit at room temperature for 15 minutes before serving, and give it a good stir after the oil melts to redistribute everything.