In a bowl, combine Greek yogurt, lemon juice, oregano, salt, and pepper. Coat the chicken breasts in the marinade and let sit for at least 30 minutes.

Boil salted water in a large pot, add pasta, and cook until al dente according to package instructions. Drain and set aside.

Heat olive oil in a skillet over medium heat. Cook marinated chicken for about 6-7 minutes per side until golden brown and cooked through. Remove from heat and slice.

In a large bowl, mix cooked pasta with sliced chicken, cherry tomatoes, cucumber, and red onion. Drizzle with olive oil and additional lemon juice.

Serve warm or chilled; garnish with fresh herbs if desired.