Place onion in bottom of a 4-quart electric slow cooker; add beef roast. Sprinkle 2 tablespoons flour, oregano, salt, and crushed red pepper over roast. Add green pepper and next 3 ingredients to slow cooker. Cover and cook on low setting 8 hours or until roast is tender.
When roast is done, cook spaghetti according to package directions, omitting salt and fat; drain well.
While pasta cooks, remove roast from cooker, and slice diagonally across grain; keep warm. Combine remaining 2 tablespoons flour and water, stirring until smooth; add to sauce in cooker. Cook, uncovered, 10 to 15 minutes or until mixture is thickened. Serve meat and sauce over spaghetti.