Preheat oven to 450 degrees F (230 degrees C). Spray a baking pan with cooking spray.
Mix olive oil and onion soup mix together in a bowl. Combine potatoes, baby carrots, bell peppers, and onion in a large bowl; add oil mixture and mix until evenly coated.
Roast in the preheated oven until vegetables are tender, 15 to 30 minutes.