Bring a large pot of lightly salted water to a boil. Add macaroni and cook for 8 to 10 minutes or until al dente; drain and cool under running water.
Mix both cans of tuna into the cooled pasta. Add the mayonnaise to your taste. You want just enough to get the flavour into the salad, but not enough to see the white of the mayonnaise on the pasta. Stir in salt, pepper, garlic powder, oregano and onion.