Heat olive oil in a large saucepan over medium high heat.
Add onion, garlic, carrot & celery and cook for a few minutes until onions are soft.
Add beef mince to saucepan, break up clumps, stir to mix through and continue cooking until mince browns.
Add diced tomatoes, tomato paste, water & oregano, stir to mix through and bring to boil.
Reduce heat and simmer, partially covered, for about 1 hour. Stir occasionally.
Remove from heat and fill cannelloni tubes per instructions below.
Cheese Sauce
Add butter to a medium saucepan over medium heat and cook until melted.
Add flour, stir to mix through until fully combined.
Increase heat and gradually add milk while continuing to stir until sauce boils and thickens.
Remove from heat, add cheese and stir to mix through.
Cannelloni
Pre-heat a fan forced oven to 160°C (320°F).
Spoon 1/4 of the filling into the base of a large round baking dish or saucepan. Ideally the dish will be approx 26cm in diameter and approx 11cm deep.
Press the cannelloni tubes, upright, into the filling. Cover the base of the baking dish.
Spoon the remainder of the filling into the cannelloni tubes. Spread as evenly as possible.
Pour the cheese sauce over the cannelloni then sprinkle with mozarella.