Prepare the Vegetables: In a large bowl, combine the mixed greens, halved cherry tomatoes, diced cucumber, and thinly sliced red onion.
Add Olives and Feta: Gently fold in the sliced Kalamata olives and crumbled feta cheese to the bowl with the vegetables.
Make the Dressing: In a separate small bowl, whisk together the olive oil, red wine vinegar, salt, and pepper until well combined.
Toss the Salad: Pour the dressing over the salad and toss everything together until it's evenly coated.
Serve: You can serve it immediately or let it chill in the refrigerator for about 30 minutes for the flavors to meld.