Fry bacon in a large pan until crisp and remove the browned bits to a bowl. Add the onion, celery, and potatoes to the hot bacon fat and sauté until soft. Remove to the bowl with the bacon.
Add a tablespoon of cooking oil to the pan if necessary, bring up to heat, add the oysters, slightly brown and remove to the bowl. Add one more tablespoon of cooking oil to the pan and add flour, stirring constantly until the flour turns a dark brown color.
Pour all the contents of the bowl into the pan and stir until coated with the browned flour (roux). Pour in the reserved oyster liquor along with three cups of milk and stir until thick. Add more milk if necessary. Salt and pepper to taste. Just before serving add a stick of butter, allow to melt, stir into the stew, and serve.