In a bowl, combine olive oil, red wine vinegar, chopped onion, minced garlic, fresh oregano, salt, and pepper to create a marinade. Cut the tri-tip steak into bite-sized cubes and add it to the marinade, ensuring all pieces are well-coated. Cover and refrigerate for 30 minutes.

While the beef marinates, soak wooden skewers in a bowl of water for about 20 minutes. This step helps prevent burning during grilling.

Preheat your grill to high heat, aiming for around 400°F. Thread the marinated beef cubes onto the soaked skewers, ensuring they are spaced evenly. Grill for 6 to 7 minutes, turning every minute.

As the beef kabobs grill, whisk together the remaining olive oil, red wine vinegar, salt, and pepper in a small bowl to create a dressing.

On a large platter, layer the chopped romaine lettuce as the base. Add sliced cucumbers, halved grape tomatoes, and sliced red onion. Crumble the feta over the salad and drizzle with dressing.

Once the beef kabobs are ready, arrange them on top of the Greek salad and serve immediately.