Make the dressing:
In a small bowl, whisk together olive oil, vinegar, garlic, oregano, mustard, salt, and several grinds of pepper. Season to taste
On a large platter, arrange the cucumber, green pepper, cherry tomatoes, feta cheese, red onions, and olives.
Drizzle with the dressing and very gently toss.
Sprinkle with a few generous pinches of oregano and top with the mint leaves.