Drain the tuna. Peel onion and cut into rings. Clean, wash and slice the tomatoes. Grease half of a baking tray. Roll out the pizza dough and spread with strained tomatoes. Cover pizza with tuna, onion rings, tomatoes and olives and season with salt, pepper and oregano. Sprinkle with cheese and bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 20 minutes