Start a large pot of generously salted water to a boil. Add the pasta and boil until al dente (check a few minutes before the package instructions indicate). Reserve about 1/2 cup pasta water, then drain the pasta and return it to the pot. Toss with a little olive oil if the sauce is not yet ready.
Meanwhile, heat a large pan over medium low heat. Melt the butter, then add the garlic and oregano and cook for 30 seconds to 1 minute until the garlic is lightly browned and fragrant. Add the milk, heavy cream and Parmesan cheese and cook for 2 to 3 minutes until the cheese has melted. Add the spinach and salt and cook 2 to 3 minutes more, until the spinach is wilted.
In a small bowl, mix the flour with 1 tablespoon pasta water. Pour it into the pan with the sauce, then stir until it thickens.
Add the drained pasta to the sauce and toss. If necessary, add a few splashes of pasta water as necessary to make a creamy consistency. Top with fresh ground black pepper and serve. Enjoy immediately; leftovers last refrigerated for up to 3 days.