Cook spaghetti in a large pot of well-salted boiling water according to package directions. Drain and rinse under cold water.
Mix the Salad
In a large bowl, combine the cooled spaghetti with the tomatoes, cucumber, bell pepper, cheese, red onion, and olives.
Add the Dressing
Pour about ¾ of the bottle (12 ounces) of Italian dressing over the mixture. Toss everything until well coated. Save the extra dressing in the fridge for later.
Chill
Cover and refrigerate for at least 2 hours (or overnight) to let the flavors come together. Just before serving, give everything a good stir. Taste and season with salt and pepper, if desired. Add the reserved dressing if the salad seems a little bit dry. Sprinkle with fresh parsley.