Drain the tuna and chop it up in a bowl, the fastest way is to take a fork and scrape the tuna out of the can. Cut the red onion into small pieces and add to the bowl. Cut the feta into small cubes and add to the bowl. Drain and add the olives.
Whip up the vinegar and oil and season well with salt, pepper and oregano. Pour the dressing over the salad ingredients and mix everything carefully.