In a large bowl, combine the cherry tomatoes, cucumber, red onion, olives, and parsley.
Drizzle with lemon juice and olive oil, then sprinkle with oregano. Toss to combine.
Add the cooked warm rice to serving bowls.
Top each bowl with chunks of tuna.
Spoon the prepared veggie mixture over the tuna and rice.
Season with salt and pepper to taste.
Garnish with crumbled feta cheese if desired, and serve immediately.