Melt butter in a large Dutch oven. Add celery and cook 3 minutes.
Add mushrooms and white and light parts of green onions. Cook until mushrooms are golden brown.
Sprinkle flour into Dutch oven. Cook and stir 1 minute.
Gradually whisk in milk and half-and-half.
Stir in potato soup and the liquid from the container of oysters. Add salt, white pepper, and Creole seasoning. Gently simmer for 10 minutes.
Add the oysters, cheddar cheese, and green parts of green onion and cook just until oysters start to curl around the edges. This will only take 2 to 3 minutes. Remove from heat and serve.
Notes
Note; It helps if the milk and half-and-half aren't cold when you add them to the pot. I measure them in a glass measuring cup and microwave them for about 20 seconds.