Heat the oil in a large nonstick frying pan over moderate heat. Add the capsicums and cook for 5 minutes or until crisp-tender. Add the mushrooms, oregano and a pinch of salt, then cook for 4 minutes or until the mushroom liquid has evaporated. Put aside.
Preheat the oven to 220°C. Spray a baking tray with cooking spray. Place the pizza base on the prepared tray. Spoon the tomato sauce over the pizza base then top with the capsicum mixture, the mozzarella and a little pepper. Bake the pizza for 15 minutes or until the cheese has melted and the crust is browned.