Place the cooked rice or quinoa into a large serving bowl.
Add the chopped cucumber, halved cherry tomatoes, and sliced red onion on top of the rice or quinoa.
Place the cooked and sliced chicken breasts over the vegetables.
Sprinkle the crumbled feta cheese and sliced olives evenly over the bowl.
In a small bowl, mix together the olive oil, lemon juice, dried oregano, salt, and black pepper.
Pour the dressing evenly over all the ingredients.
Gently toss everything together if desired, or leave the ingredients arranged for a beautiful presentation.