In a large bowl combine olive oil, vinegar, oregano, chili powder, garlic salt, salt, pepper and sugar. To the marinade add the zucchini, yellow squash, onion, green pepper and red pepper. Marinate vegetables in the refrigerator for at least 30 minutes, but not more than 24 hours.
Heat oil in a large skillet over medium-high heat. Drain the vegetables and saute until tender, about 10 to 15 minutes. Stir in the corn and beans; increase the heat to high for 5 minutes, to brown vegetables.