In a large pot or deep skillet, heat olive oil over medium heat.
Add the chopped onion and cook for 3-4 minutes until softened.
Add the minced garlic and red pepper flakes, cook for 1 minute until fragrant.
Pour in the diced tomatoes with their juice, water, dried oregano, dried basil, salt, and black pepper. Stir to combine.
Add the dried spaghetti, pressing it down gently so it is mostly submerged in the liquid.
Bring the mixture to a boil, then reduce heat to medium-low and simmer uncovered for 10-12 minutes, stirring occasionally, until the pasta is cooked and most of the liquid is absorbed.
Stir in the drained tuna gently to combine and heat through for 2-3 minutes.
Remove from heat and stir in the grated Parmesan cheese and chopped fresh parsley.
Serve immediately, garnished with additional Parmesan if desired.