Cook the chicken: Preheat the grill to medium heat. Season chicken breasts with salt and pepper. Grill for 6-7 minutes on each side until fully cooked. Let rest before slicing.
Boil the pasta: In salted boiling water, cook pasta according to package instructions until al dente (about 8-10 minutes). Drain and rinse under cold water.
Chop the veggies: Dice cherry tomatoes, cucumber, and red onion into bite-sized pieces.
Make the dressing: Whisk together olive oil, lemon juice, oregano, salt, and pepper in a small bowl.
Combine everything: In a large bowl, mix sliced chicken, cooled pasta, veggies, and feta cheese. Pour dressing over and toss gently to combine.
Serve: Enjoy immediately or chill in the fridge for later.