Put oven rack in middle position and preheat oven to 400°F.
Trim ends of mushroom stems and carefully separate caps and stems, reserving both. Arrange caps, stemmed sides up, in a buttered 13- by 9-inch shallow baking dish.
Finely chop stems, then cook with garlic and oregano in butter in a 10-inch heavy skillet over moderate heat, stirring frequently, until lightly browned, about 5 minutes. Stir in pecans, 1/4 teaspoon salt, and 1/8 teaspoon pepper and cook, stirring frequently, 1 minute. Stir in 1/3 cup cream and bring to a simmer, then remove from heat.
Sprinkle insides of caps with remaining 1/4 teaspoon salt and 1/8 teaspoon pepper, then divide filling among caps. Drizzle mushrooms with remaining 2/3 cup cream and bake until filling is browned and caps are tender, about 30 minutes. Serve mushrooms drizzled with pan juices.
Cooks' note: Stuffed mushrooms can be assembled, but not baked, 1 day ahead and chilled, covered. Bring to room temperature before baking.