If using wooden skewers, soak them in water for at least 30 minutes to prevent burning.
In a mixing bowl, combine olive oil, balsamic vinegar, dried oregano, garlic powder, salt, and pepper. Whisk until well blended.
Add the sliced zucchinis, bell pepper chunks, onion wedges, cherry tomatoes, and mushrooms to the bowl. Toss the vegetables in the marinade until they are evenly coated. Let sit for about 10 minutes to absorb the flavors.
Preheat your grill to medium-high heat (about 400°F/200°C).
Thread the marinated vegetables onto the skewers, alternating the types for a colorful presentation.
Place the vegetable skewers on the grill. Cook for 10-15 minutes, turning occasionally, until they are tender and have nice grill marks.
Remove from the grill and let rest for a few minutes before serving.