Cook the rotini pasta according to package directions until al dente, then drain and rinse under cold water to stop the cooking process and cool the pasta completely.

While pasta cooks, season chicken breast with salt, pepper, and oregano, then grill or sauté until fully cooked and internal temperature reaches 165°F, then dice into bite-sized pieces.

Prepare all vegetables by halving the cherry tomatoes, dicing the cucumber into small cubes, slicing the black olives, finely dicing the red onion, and chopping the fresh parsley.

In a small bowl, whisk together mayonnaise or Greek yogurt, olive oil, lemon juice, minced garlic, dried oregano, salt, and black pepper until smooth and well combined.

In a large mixing bowl, combine the cooled pasta, diced chicken, cherry tomatoes, cucumber, black olives, red onion, and chopped parsley.

Pour the prepared dressing over the pasta mixture and toss everything together gently but thoroughly until all ingredients are evenly coated with the creamy dressing.

Add the crumbled feta cheese and fold it in gently to avoid breaking it down too much, allowing some larger chunks to remain for texture.

Cover the bowl and refrigerate for at least one hour before serving to allow the flavors to meld together, or serve immediately if preferred.