Turn oysters into a strainer; set aside to drain.
Wipe mushrooms with a damp cloth; remove stems.
Chop mushroom stems very fine.
In a medium bowl, combine chopped mushroom stems, onions, sour cream and breadcrumbs.
Chop drained oysters very fine.
Add to breadcrumb mixture.
Preheat oven to 375°F (190°C).
Spoon oyster mixture evenly into mushroom caps.
Arrange filled mushrooms over bottom of a 13" x 9" baking dish, filling-side up.
Pour 1/2 cup water around but not on mushrooms.
Bake 15 to 20 minutes in preheated oven until mushrooms are tender.