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; let sit for 10-15 minutes.
Preheat the grill or grill pan to medium-high heat.
Thread shrimp onto skewers or place directly on the grill grates; cook for 2-3 minutes per side until opaque and slightly charred.
In a large salad bowl, combine mixed greens, cherry tomatoes, cucumber, and red onion.
Add the grilled shrimp and diced avocado to the salad base.
Drizzle with remaining olive oil or preferred vinaigrette and toss gently before serving.