Mix olive oil and ranch dressing mix together in an 8-inch square baking pan. Add 1/2 of the potatoes, 1/2 of the zucchini, 1/2 of the onion, and 1/2 of the onion; stir to coat. Add remaining potatoes, zucchini, celery, and onion; stir to coat. Sprinkle bread crumbs and garlic salt over the top.
Bake in the preheated oven until vegetables are tender, 50 to 60 minutes.