In a large mixing bowl, combine olive oil, dried oregano, fresh lemon juice, and minced garlic. Cut beef sirloin into even cubes and add to marinade, seasoning with salt and pepper. Cover and refrigerate for at least 30 minutes, or up to 3 hours.

If using wooden skewers, soak them in water for 30 minutes. After marinating, thread the beef onto skewers.

Preheat the grill to medium-high heat (around 400°F). Let coals burn until covered with ash if using a charcoal grill.

Place skewers on preheated grill. Cover and cook for about 10 minutes, turning every 2 minutes. Aim for an internal temperature of 135°F for medium-rare.

In a food processor, combine olive oil, golden balsamic vinegar, fresh mint and basil leaves, and garlic. Pulse until finely chopped. Season with salt and red pepper flakes.

Remove kabobs from the grill and let rest for a few minutes. Serve with chimichurri drizzled over.