Take a pan add olive oil and butter in it,heat it and add sliced onion and garlic cloves

Make a paste,add all-purpose flour, milk,black pepper powder, salt,oregano, chilli flakes in bowl. Mix well

Boil Spaghetti in pan add water and salt in it. Cook till soft.
Once Spaghetti is cooked, use tongs or strainer ladle to trasfer the Spaghetti in it.

Now add white sauce in it.
Bring the sauce to simmer on a medium low flame. You will notice the sauce starts to thicken.