The Absolute Best Substitute For Oyster Sauce - Tasting Table
Briefly

"Fish sauce is the best substitute for oyster sauce," she said. "But fish sauce is salty, and more pungent, so use less when [substituting] and add a little sugar."
In some dishes, you could use equal parts of the two ingredients for an even substitute if you're mindful of the salt levels. Fish sauce is also more liquidy so add a thicker like a cornstarch slurry to obtain a similar consistency."
Oyster sauce has a rich flavor that adds a sweet-and-salty umami essence to a variety of dishes from vegetables to steak marinades.
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