In a small bowl or jar, whisk together the olive oil, lemon juice, dried oregano, salt, and pepper. Set aside.
In a large mixing bowl, combine the flaked tuna, diced cucumber, halved cherry tomatoes, Kalamata olives, sliced red onion, and chopped parsley. Toss gently to mix.
Pour the dressing over the salad and toss until everything is evenly coated. Gently fold in the crumbled feta cheese, if using.
Transfer the salad to a clear glass serving bowl. Garnish with extra parsley or a sprinkle of oregano if desired. Serve immediately or refrigerate for 30 minutes to allow flavors to meld.