Cook spaghetti according to package directions. Meanwhile, cook bacon in a large skillet over medium-high heat until golden brown. Add onion and garlic and saute 3 minutes, until onion is soft. Add oregano and cook 1 minute. Add flour and stir to coat onion. Add milk and bring to a simmer. Cook 5 minutes, until sauce thickens, stirring frequently.
Remove from heat and stir in sour cream, Parmesan and parsley. Season, to taste, with salt and black pepper. Drain pasta and toss with sauce to coat. Serve.