In a bowl, combine the olive oil, minced garlic, dried oregano, salt, and pepper.
Add the beef sirloin cubes to the bowl and toss to coat them in the marinade. Let marinate for at least 10 minutes.
Preheat the grill to medium-high heat.
Thread the marinated beef cubes onto skewers, alternating with chunks of bell peppers and red onion.
Grill the skewers for about 5 minutes per side, or until the beef is cooked to your desired level of doneness.
While the skewers are grilling, warm the pita breads on the grill for a minute on each side.
Serve the grilled beef and pepper skewers with warm pita breads.
Enjoy!