Combine the olive oil and garlic in a large skillet. Cook over medium heat until the garlic is golden around the edges, about 3 minutes. Add the onion, oregano and 1 teaspoon salt. Cook, stirring, until the onion is soft but not browned, about 10 minutes. Add the tomatoes and their juices and 1/2 cup water; continue cooking until the sauce is slightly reduced, about 20 minutes. Stir in the basil and season with salt. Keep warm over low heat.
Meanwhile, bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Reserve 1 cup cooking water, then drain.
Add the pasta to the sauce along with the butter and 1/2 cup of the reserved cooking water. Increase the heat to medium and toss to coat, adding the remaining cooking water as needed to loosen the sauce. Transfer the pasta to bowls and drizzle with olive oil.