Heat oven to 450° F. In a large, ovenproof skillet or baking pan, combine the olive oil, onion, garlic, olives, anchovies (if desired), pepper flakes, and oregano. Place on low heat and bring to a simmer, stirring, 2 minutes. Remove from heat, pour the tomatoes over top, and season with salt and freshly ground pepper. Transfer to oven and roast 20 minutes until sauce is hot and bubbly and onions are soft.
Meanwhile, cook spaghetti according to package directions, about 10 to 12 minutes.
Transfer the sauce ingredients to the bowl of a food processor or blender and pulse once or twice (sauce should be chunky). Drain spaghetti and toss with the tomato sauce. Drizzle with a little more olive oil and sprinkle with parsley, if desired.