Heat oven to 450° F. In a small saucepan over medium heat, combine the tomatoes and their juices, oregano, and ¼ teaspoon salt. Bring to a boil.
Reduce heat and simmer, crushing the tomatoes with a wooden spoon until the sauce thickens, 10 to 15 minutes.
Shape the dough into one 12- to 14-inch round and place on a baking sheet. Cover with the sauce and top with the mozzarella and spinach.
Roll the pie into the shape of a log. Turn it so the seam is facing the pan and not visible. Bake until golden brown and crisp, 20 to 25 minutes.
Transfer to a cutting board. Slice into rounds and divide among plates.