Preheat the grill or a grill pan over medium-high heat.
In a small bowl, mix olive oil, oregano, garlic powder, salt, and pepper. Rub this mixture all over the chicken breasts.
Grill the chicken for about 6-7 minutes on each side, or until the internal temperature reaches 165°F. Remove from the grill and let it rest for 5 minutes before slicing.
In a large bowl, combine the cooked quinoa, cherry tomatoes, cucumber, red onion, Kalamata olives, and parsley. Toss gently to mix.
To assemble the bowls, divide the quinoa mixture among four bowls. Top each with sliced grilled chicken and a generous sprinkle of feta cheese.
Drizzle tzatziki sauce over the top before serving.