Place the mushrooms in a shallow dish. Combine the oil, garlic, oyster sauce and ketcup manis in a bowl, mix well to combine. Pour the marinade over the mushrooms and stir to coat. Cover and set aside for 10 minutes.
Preheat a greased barbecue plate or non-stick frying pan on medium. Cook the sausages, turning frequently, for 10 minutes or until golden and cooked through. Remove to a plate, cover to keep warm.
Increase the heat to high and add the marinated mushrooms. Cook, tossing frequently for 3-4 minutes or until mushrooms are tender.
Cut the baguette and sausages in half lengthways. Top the baguette with salad leaves, a sausage and a heaped spoonful of mushrooms. Serve immediately.