Place the spaghetti, clams with their juices, olive oil, oregano leaves, crushed red pepper, garlic, onion and a good pinch of salt in a large high-sided skillet. Add 4 1/2 cups of water and bring to a boil. Cook, stirring and turning the pasta frequently with tongs, until the spaghetti is al dente and most of the liquid has evaporated and thickened, 9 to 10 minutes.
Remove the pan from the heat and fold in the spinach, Parmesan and butter. Toss to make sure everything is well coated and combined. Season to taste with black pepper and more salt if needed. Serve, garnished with more oregano leaves and crushed red pepper flakes.