Clean and finely slice the mushrooms.
Juice the oranges. Mix 1 tbsp orange juice with the tomato paste and oregano.
Season the tomato sauce with salt and pepper. Spread over the sliced breads and top with mushrooms.
Cut the ham into thin strips.
Wash the basil, shake it dry and cut the leaves into strips.
Place the pizzas on a baking tray lined with baking paper and top with ham and basil. Sprinkle with cheese and bake in a preheated oven at 350°F (325°F fan, gas 4) for 8-10 minutes until golden brown.