In a small bowl or jar, whisk together olive oil, lemon juice, red wine vinegar, minced garlic, oregano, Dijon (if using), salt, and pepper until well combined. Set aside to let the flavors meld.
In a large salad bowl, combine romaine or mixed greens, diced cucumber, tomatoes, sliced red onion, and bell pepper.
Add the Kalamata olives and crumbled feta cheese over the top.
Drizzle the lemon vinaigrette over the salad and toss gently until everything is evenly coated with dressing.
Serve immediately so the crisp vegetables stay fresh and crunchy.