Heat the oil in a medium-size saucepan over low heat. Add the onion and garlic; cook, stirring occasionally, for about 10 minutes or until softened. Add the diced tomatoes, capers, olives and oregano. Increase the heat to medium; cook, stirring occasionally, for 30 minutes. Add the tuna and heat through.
Cook the pasta according to package directions until al dente, firm but tender. Drain well.
Spoon the sauce over the hot cooked pasta on a large serving platter. Sprinkle with the parsley.