In a medium bowl, whisk together 2 tablespoons of olive oil, lemon juice, minced garlic, oregano, salt, and pepper.
Add the shrimp to the marinade and toss to coat thoroughly; let sit at room temperature for 10 minutes.
Preheat a grill or grill pan over medium-high heat (approximately 400°F/200°C).
Grill the shrimp for 2 to 3 minutes per side until opaque, pink, and slightly charred.
In a large bowl, combine the mixed greens, cherry tomatoes, cucumber, and red onion.
Whisk together the remaining 3 tablespoons of olive oil, red wine vinegar, and Dijon mustard to create the dressing.
Toss the salad components with the dressing until evenly coated.
Divide the salad onto four plates, top with the grilled shrimp, and garnish with crumbled feta cheese.