Thick, nearly black soy sauce that tastes sweeter and less salty than light soy, made by cooking soy sauce with caramel. A little gives red-braised pork, fried rice and noodles a glossy deep brown.
Always paired against 生抽 (light soy). Old soy is thicker and darker, used mainly for color in braising; light soy is used for seasoning. Mixing them up turns a dish muddy.