In a small bowl, whisk together the olive oil, lemon juice, dried oregano, salt, and black pepper to create the dressing.
In a medium mixing bowl, combine the drained tuna and cannellini beans.
Add the diced red onion, cucumber, and chopped parsley to the bowl.
Pour the dressing over the tuna and bean mixture.
Toss gently to combine, breaking up larger chunks of tuna as desired.
Serve immediately or refrigerate for up to 30 minutes to allow flavors to meld.