Coat large nonstick skillet with cooking spray; heat over medium heat until hot. Add zucchini; cook and stir 5 minutes. Add onion, garlic and oregano; cook 5 minutes more or until zucchini and onion are lightly browned. Stir in salsa. Bring just to a boil. Reduce heat to low; simmer 5 minutes more or until zucchini is crisp-tender. Sprinkle cheese on top; cover and cook 1 to 2 minutes or until cheese melts. Serve hot.