Place shiitake mushrooms stem side up on large rimmed baking sheet. Mound each with about 1/2 tablespoon pork sausage. Stir plum sauce in heavy small saucepan over medium heat until melted.
Whisk in oil.
Brush mushrooms with all of plum sauce mixture. (Can be made 1 day ahead. Cover; refrigerate.)
Preheat oven to 400°F.
Bake mushrooms until sausage is cooked through, about 15 minutes.