What is Oyster Sauce?_Oyster sauce is a thick, brown sauce with a sweet, salty, and earthy flavor. Oyster sauce is a popular ingredient in Vietnamese, Thai, and Cantonese cuisine. This thick, flavorful sauce is full of umami, which gives any recipe it's added to extra oomph. What is in Oyster Sauce? Traditionally, oyster sauce is made by slowly simmering oysters in water until the juices caramelize into a thick, brown, intensely flavorful sauce.