Start by bringing a pot of salted water to a boil for the pasta.
Set aside four anchovy fillets for garnish, then pour the oil from the rest of the can into a 5-quart sauté pan and warm it over medium heat.
Add garlic and the remaining anchovies to the heated oil. Stir frequently until the garlic softens and the anchovies dissolve, which will take approximately one minute.
Add the tomato paste and red pepper flakes, cooking for another minute to blend the flavors.
Incorporate the crushed tomatoes, olives, capers, and oregano into the mixture.
Allow the sauce to reach a gentle boil, then reduce the heat to low and let it simmer uncovered for 15 to 20 minutes.
While the sauce simmers, add the pasta to the boiling water and cook as per the package instructions until al dente.
Cut the tuna into bite-sized chunks and gently stir them into the sauce. Allow it to cook for a few more minutes until the tuna is heated through.
Once the pasta is cooked, drain it well. Serve the pasta with the tuna sauce spooned over the top.
Finish off by topping the dish with the reserved anchovies and a garnish of fresh parsley.