Preheat your grill to medium-high heat.
In a large bowl, combine the zucchini, yellow squash, bell peppers, red onion, and mushrooms.
In a small bowl, whisk together the olive oil, balsamic vinegar, garlic powder, oregano, salt, and black pepper.
Pour the dressing over the vegetables and toss until they are evenly coated.
Place the vegetables in a grill basket or directly on the grill grates. Grill for about 10-15 minutes, turning occasionally, until the vegetables are tender and have nice grill marks.
Remove the vegetables from the grill and serve warm.