Heat a Dutch oven or large saucepan over medium-high heat. Add oil; swirl to coat. Add onion and garlic to pan; sauté 3 minutes or until onion starts to brown. Add tomatoes, stock, oregano, and pasta, in that order. Bring to a boil. Add noodles; stir to submerge in liquid. Cover, reduce heat to medium-low, and cook 7 minutes or until pasta is almost done. Uncover; stir in salt. Add spinach in batches, stirring until spinach wilts. Remove from heat; let stand 5 minutes. Sprinkle with cheese and serve.