1FOR THE LEEKS: Cut leeks crosswise into 2-inch lengths, then cut lengthwise into enough thing strips to measure 2 cups. Wash leek strips in a bowl of cold water, then lift out and pat dry.
2Heat oil in a deep 4-quart sauce until it registers 360°F. Fry leeks in several batches, stirring, until golden, about 10 seconds per batch. Transfer sautéed leeks with a slotted spoon to paper towels to drain. Cool completely (leeks will crips as they cool.)
3FOR THE SOUP: Peel potatoes and cut into 1/2-inch cubes. Cook leeks, potatoes, and salt in butter in a 4-quart heavy saucepan, covered, over low heat, stirring occasionally, until leeks are golden and potatoes are beginning to soften, about 15 minutes. Add water, cover saucepan, and simmer over moderate heat until potatoes are very tender, about 10 minutes.
4Puree soup in batches in a blender until very smooth.
5Once pureed, return pureed mixture to saucepan. Add oyster liquid and half-and-half, and bring to a simmer over moderate heat, stirring occasionally (do not boil). Add oysters, salt, pepper and nutmeg and cook, stirring occasionally about 3 minutes or until oysters become plump and their edges curl.