Greek Salad

Name: Greek Salad
Ingredients: 3 Tbsp. extra-virgin olive oil; 3 Tbsp. red wine vinegar; 2 tsp. Dijon mustard; 1 tsp. dried oregano; Kosher salt, freshly ground pepper; 1 large cucumber, coarsely chopped; 1 medium green bell pepper, ribs and seeds removed, thinly sliced; 1 small red onion, thinly sliced; 12 oz. tomatoes, preferably a mix of sizes, halved, quartered, or sliced; ⅔ cup Kalamata olives, pitted, torn; 6 oz. feta, cut into thin slabs