For the dough: crumble yeast in a bowl, add sugar and about 200 ml of lukewarm water (approximately 3/4 cup), whisk until smooth. Combine flour, olive oil and salt, add yeast mixture and knead to a smooth dough. Cover and let rise for 1 hour in a warm place.
Divide dough into 4 balls, roll out on a floured surface into round cakes about 24 cm in diameter (approximately 10 inches), making the edges slightly thicker. Arrange on oiled baking pans.
Spread with tomatoes and season with salt and pepper. Cut mozzarella into thin slices and arrange on top of tomatoes. Sprinkle with oregano and drizzle with olive oil.
Bake in preheated oven at 220°C (approximately 425°F) for about 20 minutes or until golden brown. Remove from oven and serve.