Heat oven to 450° F.
In a large ovenproof skillet or baking pan over low heat, combine the oil, onion, garlic, olives, anchovies (if desired), crushed red pepper, and oregano.
Bring to a simmer, stirring, 2 minutes. Remove from heat, pour the tomatoes over the top, and season with ½ teaspoon salt and ¼ teaspoon pepper.
Transfer to oven and roast for 20 minutes until the sauce is hot and bubbly and the onions are soft.
Meanwhile, cook the spaghetti according to the package directions. Drain.
Transfer the roasted tomato mixture to a food processor or blender and pulse once or twice (sauce should be chunky). Toss with the spaghetti and parsley.