Make the vinaigrette first, so the flavors have a little time to blend. In a bowl or jar, combine the olive oil, red wine vinegar, lemon juice, oregano, salt, and pepper. Whisk the ingredients together, or close the jar and shake until combined. Set the vinaigrette aside.

If you want to soften the flavor of the red onion a bit, slice it then soak the slices in ice water for about five minutes before dicing. Otherwise, finely dice the red onion, cut the cucumber into quarter rounds, slice the tomatoes in half, and roughly chop the parsley.

Combine the cucumber, tomato, onion, and parsley in a bowl. Pour the vinaigrette over top, then stir until the vegetables are coated.

Rinse and drain the beans, and drain the tuna. Add the beans, tuna, and feta to the salad, then gently stir to combine. These three are added last and only gently stirred to make sure the tuna remains in larger flakes instead of falling apart into tiny pieces.

Serve immediately, or refrigerate up to four days. Give it a quick stir after refrigerating and before serving to redistribute the dressing.