01 - In a bowl, whisk together olive oil, soy sauce, red wine vinegar, minced garlic, oregano, black pepper, and salt. Add beef cubes, toss thoroughly to coat, cover, and refrigerate for at least 1 hour, up to 8 hours.
02 - Preheat grill or broiler to medium-high heat. If using wooden skewers, soak them in water for 30 minutes to prevent burning.
03 - In a large bowl, toss bell peppers, mushrooms, and red onion with 2 tablespoons olive oil, salt, and pepper until evenly coated.
04 - Thread marinated beef, bell peppers, mushrooms, and red onion alternately onto skewers, distributing ingredients evenly.
05 - Grill skewers for 10 to 15 minutes, turning occasionally, until beef reaches desired doneness and vegetables are tender with slight charring.
06 - Allow skewers to rest for 2 minutes off heat before serving hot.