While its flavor is similar to the green bean (Phaseolus vulgaris), it is from a different genus. In fact, it is a member of the cowpea species, which includes blackeyed peas.
WHAT IS OYSTER SAUCE?
A popular condiment in Chinese and Filipino cuisines, oyster sauce is often used in stir fries and as a topping for steamed vegetables.
The viscous sauce is prepared from oysters and brine. A true oyster sauce is aromatic with intense flavor, and is expensive. Most oyster sauces on the market are cheaper, diluted solutions of concentrated ones. There are also vegan versions made from mushrooms.
Oyster sauce was invented in 1888 by Mr. Lee Kam-Sheung of Nam Shui Village in Guangdong Province, Chinas, an island where oysters were abundant.
Originally born in another village, Lee was a farmer who had to leave town following threats from local gangsters. He opened a restaurant in his new village, using the local oysters to make stock. One day he forgot about a pot of stock on the stove, and when he returned to it the stock had boiled down to a thick, aromatic and delicious sauce: a happy accident!*
His company, Lee Kum Kee, continues to produce oyster sauce along with a wide variety of fine Asian condiments.
*Source: Small Business Management and Entrepreneurship in Hong Kong: A Casebook, edited by Ali F. Farhoomand. Here’s the page reference.