Step 1: In a small bowl, whisk together the olive oil, lemon juice, minced garlic, oregano, salt, and pepper to create the marinade.
Step 2: Place the steak in a shallow dish or resealable bag and pour half of the marinade over it. Reserve the other half for the salad dressing. Let the steak marinate for 30-60 minutes at room temperature.
Step 3: Preheat your grill to high heat. Remove the steak from the marinade and grill for 5-6 minutes per side, or until it reaches your desired level of doneness (135°F for medium-rare).
Step 4: Transfer the steak to a cutting board and let it rest for at least 10 minutes before slicing it against the grain into thin strips.
Step 5: In a large salad bowl, combine the mixed greens, tomatoes, cucumber, red onion, and olives.
Step 6: Toss the salad with the reserved dressing, then top with the sliced grilled steak and crumbled feta cheese. Serve immediately.