Heat the olive oil in a large nonstick skillet coated with cooking spray over medium-high heat. Add onion and garlic; sauté for 1 minute. Add zucchini; sauté for 2 minutes or until lightly browned. Remove from heat; stir in oregano, salt, and pepper.
Wipe pan clean with paper towels, and coat with cooking spray. Heat pan over medium heat. Add 1 tortilla to pan, and sprinkle with 1/4 cup mozzarella. Top with half of the zucchini mixture, 1/4 cup tomato, 2 tablespoons olives, 2 tablespoons feta, and 1 tortilla. Cook for 3 minutes or until lightly browned on bottom. Carefully turn quesadilla; cook for 2 minutes or until lightly browned. Place quesadilla on a cutting board; cut in half using a serrated knife. Repeat procedure with the remaining tortillas, mozzarella, zucchini mixture, tomato, olives, and feta. Serve warm.