Add the stale bread to a bowl of cold water and let soak until soft. Squeeze the water out of the bread and crumble into a large bowl.

Finely chop the red onion, tomatoes, and cucumber and add to the bowl. Cut the olives in half and add to the bowl along with the tuna and oregano. Mix everything together.

Add the vinegar and oil to a small bowl with a good pinch of salt and pepper, stir and pour over the bread and vegetables. Stir everything together, taste and add more salt if required.