Oyster Sauce is a thick, salty-tasting savoury sauce with a mild oyster flavour to it. It is usually made from a large oyster species called Crassostrea sp. It has no fishy taste, despite its name.
It is used a great deal in Chinese restaurants as it is a standard ingredient in Chinese cooking. It is also used in many Asian cuisines. It can be used as an ingredient or as a table condiment.
Commercially, it is sold in bottles and occasionally is available in tins. Different brands make different qualities of it.
A vegetarian version is made from oyster mushrooms.
Pure versions are very expensive. They are made from nothing but oysters and water. There are no additional ingredients, not even MSG or salt.
The oysters are first steamed or simmered to produce a broth from them. They are removed, and the broth is then simmered to concentrate the flavour and thicken it. Simmering continues until the sauce is quite thick and has browned from caramelization.
These are the versions most of us are likely to encounter.
Most commercial brands of Oyster Sauces are flavoured with oyster extract flavouring, thickened with a starch such as cornflour, coloured with caramel to make it dark (otherwise the mixture would be an unappealing grey) and flavoured boosted with MSG.
Some brands also use other ingredients such as soy sauce and sugar.
Some countries, ironically, really only allow the import of artificial Oyster Sauce with very little actual oyster content in it, owing to regulations governing the import of sea foods.
Particularly good with steamed green vegetables such as broccoli or bok choy.
Store opened bottles in refrigerator. Transfer contents of opened tins to a sealable bottle and store in refrigerator.
Oyster Sauce is not really that old; as far as Chinese cuisine goes, it is a new-comer.
It was first made in 1888 by the Lee Kum Kee company (or more precisely, by the company founder, Lee Kam Sheung) in Guangdong Province, China.
Bittman, Mark. Vegetables Dressed in Chinese Robes. New York Times. 21 October 2009.