Whisk together olive oil, lemon juice, minced garlic, oregano, salt, and pepper in a small bowl to create the marinade and dressing.
Place chicken in a shallow dish and coat with half of the prepared dressing; marinate for 15 minutes.
Heat a grill pan or skillet over medium-high heat and cook chicken for 6 to 7 minutes per side until the internal temperature reaches 165 degrees Fahrenheit.
Remove chicken from heat, let rest for 5 minutes, then slice into strips.
In a large salad bowl, combine romaine lettuce, cucumber, tomatoes, red onion, and olives.
Top the salad with the sliced chicken and crumbled feta cheese.
Drizzle the remaining dressing over the salad and garnish with fresh parsley before serving.