Leftover grilled tuna or swordfish would make a delicious substitute for the canned tuna.
In a large bowl, combine all ingredients except pasta and tuna. Stir well, cover, and let sit for 1 hour. Set aside about 1 1/2 cups.
Cook pasta in boiling salted water until al dente. Drain well. Add to large bowl of sauce along with tuna, and combine.
Serve in shallow bowls topped with some of the reserved sauce. Garnish with oregano.