In a bowl, combine the olive oil, oregano, garlic powder, smoked paprika, salt, and pepper. Mix well to form a marinade.
Place the chicken breasts in the marinade, ensuring they are coated evenly. Cover and marinate for at least 30 minutes (or up to overnight in the refrigerator for extra flavor).
Preheat a grill or grill pan over medium-high heat. Grill the marinated chicken for about 6-7 minutes per side, or until cooked through and internal temperature reaches 165°F (75°C). Remove from heat and let it rest for 5 minutes before slicing.
In the meantime, prepare the quinoa or rice according to package instructions if not pre-cooked.
To assemble the bowls, start by placing a generous scoop of quinoa or rice at the bottom of each bowl.
Top with sliced grilled chicken, diced cucumber, diced tomato, sliced red onion, halved cherry tomatoes, crumbled feta cheese, and sliced Kalamata olives.
Garnish with fresh parsley and drizzle with extra olive oil and lemon juice, if desired.