Wipe mushrooms with damp cloth; remove stems.
Chop stems very fine.
In medium bowl, combine chopped mushroom stems, onions, sour cream and breadcrumbs.
Chop drained oysters very fine and add to breadcrumb mixture.
Preheat oven to 375 degrees (200 C.).
Spoon oyster mixture evenly into mushroom caps and arrange in bottom (fill side up) of 13 inch by 9 inch baking dish.
Pour 1/2 cup (125 ml) water around but not on mushrooms.
Bake for 15 to 20 minutes in preheated oven until mushrooms are tender.