Preheat oven to 225 °C (circulating air: 200 °C/gas: see manufacturer). Drain tuna and corn well. Wash the tomatoes and cut them into thin slices. Peel onion and cut into rings.
Wash and finely chop the parsley.
Cut flat bread crosswise. Place both halves as a circle on baking paper. Spread bread with ketchup and cover with tomatoes. Season with salt, pepper and oregano. Spread tuna, corn and onion on top.
Crumble the feta and sprinkle parsley over it. Sprinkle flatbread pizza with oil. Bake in a hot oven for 12-15 minutes. Cut pizza into pieces and serve.