Bring a large pot of salted water to a boil over high heat.
Add the spaghetti and cook until al dente, about 8 minutes. Drain and set aside.
Meanwhile, heat the olive oil in a large skillet over medium heat. Add the onion and cook until softened, about 5 minutes.
Add the garlic, oregano, and red pepper flakes, and cook until fragrant, about 1 minute.
Add the crushed tomatoes and season with salt and pepper to taste. Bring the sauce to a simmer and cook for 10 minutes.
Add the cooked spaghetti to the sauce and toss to combine. Cook until the spaghetti is heated through, about 2 minutes. Serve with freshly grated Parmesan cheese, if desired.
Interesting Facts
Spaghetti originated in Sicily and is one of the most popular Italian dishes.
Tomatoes were not added to spaghetti until the 18th century.
Parmesan cheese is a staple in Italian cooking and is often used to garnish dishes.