Preheat the oven to 425°F.
Heat the oil in a large non-stick skillet over medium heat.
Add the onion and cook, stirring frequently, until softened, about 4 minutes.
Stir in the garlic, oregano, and salt; cook, stirring, about 1 minute.
Stir in the spinach and cook until hot, about 2 minutes.
Remove the skillet from the heat and stir in the feta cheese.
Arrange the pitas on a baking sheet.
Top with the spinach mixture, spreading it to the edges.
Arrange the tomatoes on top.
Bake until the pitas are crisp, about 8-10 minutes.
Sprinkle the pizzas with the mozzarella cheese; bake until melted, about 3 minutes.