For the soup: Heat the oil in a large saucepan over high heat and saute the potatoes, celery and onions. Add 6 cups water (1 1/2 liters) and the cream and season with salt and pepper. Add the reserved oyster juices and bring the mixture to a boil. Reduce the heat and simmer until the vegetables are nice and tender, about 30 minutes.
Puree the soup with an immersion blender or in a regular blender until smooth (be careful when blending hot liquids). Pass through a fine sieve if necessary. Stir in the butter and chives. Add the oysters just before serving to prevent them from overcooking. Season with more salt and pepper if desired.
For the bacon and vegetable garnish: Brown the bacon in a skillet over medium-high heat for 3 minutes. Add the celery and onions and saute until soft, about 2 minutes. Add the potato and cook for about 5 minutes.
To serve, ladle the soup into bowls and add the bacon and vegetable garnish. Drizzle with olive oil and garnish with chervil.