Bring a large pot of salted water to a boil. Add the spaghetti and cook as the label directs. Reserve 1/2 cup cooking water, then drain.
Meanwhile, cook the bacon in a large pot or Dutch oven over medium-high heat until browned, about 7 minutes. Pour off all but 2 tablespoons drippings from the pot.
Add the mushrooms and red onion to the pot with the bacon and season with salt. Cook, stirring occasionally, until the vegetables are softened, about 4 minutes. Add the garlic and red pepper flakes and cook until the garlic is golden, about 1 minute. Add the tomatoes, oregano and 1/2 teaspoon salt. Cook, stirring occasionally, until the sauce is slightly thickened, about 6 minutes.
Add the spaghetti and reserved cooking water to the pot; cook, tossing, until coated, about 2 minutes. Add the parmesan, season with salt and toss. Drizzle each serving of pasta with the olive oil and top with parsley and more parmesan.