Melt the butter in a heavy-bottomed skillet or wok over medium-high heat; cook and stir the celery and garlic in the butter until the celery is soft, 5 to 7 minutes.
Add the flour and stir, scraping the bottom of pan until flour is lightly toasted. Slowly add the milk and half-and-half to the roux, stirring constantly. Stir the onion powder, parsley, and salt into the mixture. Reduce heat to medium-low; continue cooking and stirring until the mixture bubbles and thickens, 5 to 7 minutes.
Add the oysters with the liquid from the container to the mixture; cook until the edges of the oysters curl, about 5 minutes more. Season with pepper just before serving.