Cook pasta according to package directions, omitting salt and fat. Drain well; set aside.
Heat oil in a large nonstick skillet over medium heat until hot. Add pancetta; sauté 3 minutes. Stir in onion, 1/4 teaspoon salt, and 1/8 teaspoon pepper; cover and cook 15 minutes, stirring frequently. Uncover and cook an additional 10 minutes, stirring frequently. Remove onion mixture from skillet; set aside.
Add mushrooms, 1/4 teaspoon salt, and 1/8 teaspoon pepper to skillet; cook over medium-high heat 4 minutes. Add oregano and 2 tablespoons parsley; cook 1 minute. Add garlic; sauté 1 minute. Add onion mixture and broth; cook until thoroughly heated. Add mushroom mixture to pasta; toss to coat. Divide among 4 shallow bowls; sprinkle evenly with 2 tablespoons parsley. Garnish with oregano, if desired.