In a large pot or deep skillet, heat the olive oil over medium heat.
Add the chopped onion and sauté for 3-4 minutes until softened and translucent.
Add the minced garlic and cook for another 1 minute until fragrant.
Pour in the diced tomatoes with their juice, water, dried oregano, red pepper flakes, salt, and black pepper. Stir to combine.
Add the dried spaghetti to the pot, pressing it down gently to submerge in the liquid as much as possible.
Bring the mixture to a boil, then reduce the heat to medium-low and simmer uncovered, stirring occasionally, for about 10-12 minutes or until the pasta is cooked al dente and most of the liquid is absorbed.
Once the pasta is cooked, stir in the drained tuna, breaking it up gently with a spoon to distribute evenly.
Cook for an additional 2 minutes to heat the tuna through.
Remove the pot from heat and stir in the chopped fresh parsley and grated Parmesan cheese.
Adjust seasoning with additional salt and pepper if needed.
Serve immediately, garnished with extra Parmesan and parsley if desired.