Heat a grill to medium-high. Meanwhile, trim ends of zucchini and summer squash and cut on the bias into rounds about 1/2-inch thick and 3 inches long. Place in large bowl and toss with oil, using your hands to evenly coat all sides. Season to taste with salt and pepper and toss again.
Place vegetables in a single layer on grill and cook until browned with grill marks, 4 to 5 minutes on each side. Transfer to platter and serve hot, warm, or at room temperature.