Drain the tuna thoroughly and transfer to a medium mixing bowl.
Flake the tuna using a fork into bite-sized chunks.
Add the diced cucumber, halved cherry tomatoes, red onion, olives, and parsley to the bowl.
In a small separate bowl, whisk together the olive oil, lemon juice, dried oregano, salt, and pepper until emulsified.
Pour the vinaigrette over the tuna mixture and toss gently with a spoon until all ingredients are evenly coated.
Taste and adjust seasoning if necessary; serve chilled or at room temperature.