Preheat the oven to 180°C (350°F).
Cook the pasta in a large pot of salted boiling water according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add the chopped onion and cook for 5 minutes until softened.
Add minced garlic to the skillet and cook for 1 minute until fragrant.
Stir in the canned chopped tomatoes, dried oregano, salt, and black pepper. Simmer for 5 minutes.
Add the drained tuna to the tomato mixture and stir to combine. Remove from heat.
In a separate saucepan, melt the butter over medium heat.
Whisk in the flour and cook for 1-2 minutes to form a roux.
Gradually whisk in the milk, stirring constantly until the sauce thickens, about 5 minutes.
Remove the sauce from heat and stir in half of the grated cheddar cheese until melted and smooth.
Combine the cooked pasta with the tuna tomato mixture in a large mixing bowl.
Pour the cheese sauce over the pasta and tuna mixture and gently fold to combine evenly.
Transfer the mixture to a greased 9×13 inch (23×33 cm) baking dish.
Sprinkle the remaining cheddar cheese and the mozzarella cheese evenly over the top.
Bake in the preheated oven for 25-30 minutes until the cheese is golden and bubbly.
Remove from the oven and sprinkle fresh chopped parsley on top before serving.
To freeze: Allow the baked pasta to cool completely, cover tightly with foil or plastic wrap, and freeze for up to 3 months.
To reheat frozen tuna pasta bake: thaw overnight in the refrigerator, then bake at 180°C (350°F) for 20-25 minutes until heated through.