In a bowl, whisk together olive oil, soy sauce, balsamic vinegar, dried oregano, salt, and pepper.
Add the beef cubes to the marinade and let sit for at least 15 minutes.
Preheat the grill or grill pan to medium-high heat.
Thread the marinated beef cubes, bell peppers, onion wedges, and zucchini slices onto the skewers, alternating the ingredients.
Grill the skewers for 10-15 minutes, turning occasionally, until the beef is cooked to your desired level of doneness and the vegetables are tender.
Serve the skewers hot with a side of rice or salad.