Drain the can of tuna in water well in a sieve. Rinse the cucumber and cherry tomatoes. Cut the cucumber into cubes and halve the cherry tomatoes. Peel the red onion and finely chop it. Slice the ten black olives into rings and drain the tablespoon of capers.
Take a large bowl and put the tuna in it. Flake the fish with a fork into smaller pieces. Add the cut cucumber, cherry tomatoes, red onion, olives, and capers. Mix all the vegetables and fish together until everything is evenly distributed.
Pour a splash of olive oil and the lemon juice over the mixture. Sprinkle the oregano over it. Crumble the eighty grams of feta with your fingers into large pieces directly above the bowl. Gently toss the salad once more so the dressing is distributed but the cheese stays intact.
Divide the finished salad among the plates or spoon it into a serving dish. Serve the meal immediately so the vegetables retain their crispy texture.