Heat olive oil in a large skillet over medium heat. Add garlic and cook for 1 minute. Stir in oregano, red pepper flakes, salt and black pepper.
Add bell peppers, zucchini and mushrooms to the skillet. Cook for 8-10 minutes until vegetables are tender.
To assemble panini: Place one slice of bread on a clean work surface. Top with cooked vegetables, then top with another slice of bread.
Heat a large skillet or panini press over medium heat. Place sandwich in the skillet and cook for 2-3 minutes per side until golden brown and cheese is melted.