The Real Difference Between Fish Sauce And Oyster Sauce

Oyster sauce and fish sauce are both used in a number of cuisines to add a salty, savory, umami flavor to the dish, but although they both come from seafood products, they're quite different from one another. Traditionally, oyster sauce was made out of a reduction of the liquid in which oysters were poached (via Spiceography). However, commercially-produced oyster sauce may very well contain significantly less oyster concentrate and instead have extra ingredients such as sugar, salt, and soy sauce. It's thick and dark — it's a brownish-black color, and you may have to work a bit to get it to dissolve if it's called for in a recipe as part of a mixture. It's sweet and earthy tasting, although you would be hard-pressed to say it tastes like oysters, raw or cooked. 

Fish sauce, on the other hand, is pungent and is a staple of Thai and Vietnamese cuisines (via Cook's Illustrated). It's made from fermented fish (usually anchovies) and salt. It's a significantly thinner sauce than oyster sauce and has a reddish-brown tint to it. True oyster sauce, made primarily of oyster poaching liquid, is thinner than the mass-produced variety, but users find that it's still thicker than fish sauce.