Bring a large pot of salted water to a boil. Cook spaghetti until al dente according to package directions, usually 8-10 minutes. Drain and rinse under cold water until completely cool. Shake off excess water and set aside.

While pasta cooks, halve the cherry tomatoes, dice the cucumber and bell pepper, finely dice the red onion, and quarter the pepperoni slices. Drain the olives.

In a large mixing bowl, combine the cooled spaghetti, all chopped vegetables, pepperoni, olives, and Parmesan cheese. Toss gently to distribute evenly.

In a small bowl, stir together the Italian dressing, dried oregano, and red pepper flakes if using. This wakes up bottled dressing and makes it taste homemade.

Pour dressing over the salad and toss until everything is evenly coated. Cover and refrigerate at least 1 hour before serving, or up to 24 hours. Taste and add more dressing if needed before serving, as pasta absorbs dressing as it sits.