Heat olive oil in a large saucepan, pot or skillet over medium heat until shimmering. Add onion and garlic and teaspoon of salt. Cook, stirring occasionally, until tender but not browned.
Add crushed tomatoes (juices and all). Fill the can the tomatoes came in halfway with water. Stir the water around in the can to capture any leftover juices then pour the water into the pot. Add the oregano and stir to combine. Bring sauce to a boil then reduce heat to low and simmer.
Meanwhile, bring a large pot of heavily salted water to a boil over high heat. Add spaghetti and cook a few minutes less than the package instructions say. Drain.
Taste the sauce and season with more salt if needed. Add spaghetti into the pot with the sauce and stir to coat the pasta in the sauce. Return to simmer and let pasta finish cooking a minute or two in the sauce.
Serve in big bowls with lots of thinly sliced basil and grated parmesan cheese.