Bring a large pot of lightly salted water to a boil; cook shell pasta at a boil until tender yet firm to the bite, about 8 minutes. Drain and rinse under cold water.
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on paper towels; crumble.
Whisk creamy salad dressing, vinegar, Worcestershire sauce, olive oil, garlic, Dijon mustard, and oregano together in a bowl to make a creamy dressing.
Mix pasta, bacon, red onion, water chestnuts, eggs, green bell pepper, red bell pepper, celery, olives, Cheddar cheese, and Parmesan cheese together in a large bowl. Add dressing and stir to coat. Chill in refrigerator at least 1 hour before serving.