Bring a large pot of heavily salted water to a boil. Cook the spaghetti according to the package directions.
While the spaghetti is cooking, heat 2 tablespoons of olive oil in a large skillet over medium-high heat. When the oil is hot, add the capers and fry until crisp, 1-2 minutes. Using a slotted spoon, remove the capers and set aside.
To the oil, add the anchovy paste and cook, stirring, until it melts into the oil, about 1 minute. Add the garlic and cook, stirring, for 30 seconds. Drain the tuna, then flake it into the oil along with the tomatoes, oregano, and red pepper flakes. Add 2 ladlefuls of the pasta cooking water and simmer until the pasta is done cooking and the tomatoes break down a little and everything is heated through.
Reserve about 1 cup of the pasta cooking water, then drain. Add the spaghetti to the pan and toss to mix. If the dish seems too dry, add a little of the reserved pasta water, as needed. Season to taste with salt and pepper. Take off the heat and stir in the parsley. Top with the reserved capers and drizzle with a little olive oil.