Spread 1 tablespoon pizza sauce over each bottom focaccia half and sprinkle with oregano.
Divide half the mozzarella over the sauce, add pepperoni in one thin layer and top with remaining mozzarella. Close the sandwiches.
Brush the outside faces with olive oil and heat a large skillet over medium-low heat.
Cook for 4 to 5 minutes per side, pressing gently with a spatula and covering for 2 minutes after turning, until the focaccia is crisp and mozzarella is melted throughout.
Rest for 2 minutes, then cut each sandwich in half and serve.