Make the pizza base: In a small bowl, mix the yeast with 3 tablespoons of warm water and let sit for 15 minutes.
Sift the flour and salt into a separate deep container, make a well in it, pour in the yeast mixture, the remaining water and start kneading the dough. When it becomes homogeneous, pour in some of the oil and knead for about 10 more minutes until the dough becomes smooth and elastic.
Form a ball from the dough, grease with the remaining oil and cover the container with cling film. Leave in a warm place for 1 hour until it doubles in volume.
Place the dough on a floured work surface and knead lightly. Roll into a thin flat cake with a diameter of 25-30 cm and place on a baking sheet covered with baking paper.
Grease the base with tomato sauce, lay out the mushrooms, cut into thin slices, sprinkle everything with cheese, oregano and drizzle with olive oil.
Bake the pizza for 10-15 minutes, then cut into portions and serve.