Boil a large pot of salted water and cook the pasta according to package directions until al dente.
Drain the pasta completely and rinse under cold water to stop the cooking process.
Heat a skillet with one tablespoon of olive oil over medium-high heat.
Season the diced chicken breasts with salt, pepper, and dried oregano.
Sear the chicken until golden brown and cooked through, then set aside to cool completely.
In a small jar, whisk together olive oil, lemon juice, Dijon mustard, and minced garlic until emulsified.
In a large mixing bowl, combine the cooled pasta, cooked chicken, tomatoes, cucumber, olives, and feta cheese.
Pour the dressing over the salad and toss well to coat all ingredients evenly.
Serve immediately or store in the refrigerator to allow flavors to meld.