Heat up a large stockpot on medium high, and add diced bacon. Fry until browned but just shy of crispy. This took me around 7-10 minutes. Drain all but about 2-3 tbsp of the bacon fat off.
Add the chopped potato, carrot, and cabbage. Stir for 2-3 minutes in the bacon fat. Add a pinch of salt and pepper.
Add the stock and bring to the boil. Reduce heat to a simmer, and simmer with the lid on for around 15 minutes.
Add the chopped leeks (white part only), stir, cover and simmer for 15 minutes.
Stir in oatmeal for the last 5 minutes of cooking, or however long your package directions indicate cooking time for the oatmeal.
Taste, and adjust for seasoning. Ladle into bowls and garnish with parsley.