In a small bowl, whisk the EVOO, vinegar, garlic and oregano; season with salt and pepper.
Preheat a grill to medium-high. In a small bowl, gently toss the beef with 2 tbsp. of the dressing. In a large bowl, toss the remaining dressing with the tomatoes, cucumber, onion and olives.
Threat the meat onto 4 metal skewers (or 4 bamboo skewers presoaked in water); season with salt and pepper. Grill the kebabs until browned on all sides, about 5 minutes total for medium-rare.
Add the spinach to the vegetables and toss to combine. Divide the salad among 4 plates and serve with the kebabs.