In a bowl, whisk together olive oil, lemon juice, garlic, oregano, salt, and pepper.
Add chicken and marinate for at least 30 minutes, preferably longer.
Grill or pan-sear chicken over medium heat until cooked through, about 6-7 minutes per side.
Let chicken rest, then slice into strips.
Divide cooked rice or quinoa into bowls.
Top with chicken, tomatoes, cucumber, onion, olives, feta, and parsley.
Drizzle with tzatziki sauce and serve immediately or store for meal prep.