As far as I know, there is not, since oyster sauce contains Extract of Oyster.
OUKosher note: The following OU certified companies make kosher artificial flavor oyster sauce (as listed on the OUKosher Product Search) -
Jen Mai Oyster Sauce (Industrial)
Kosher Depot Oyster Flavor Sauce (Consumer)
Lotus Blossom Foods Oyster Flavor Sauce (Consumer)
Sugar Mill Oyster Sauce (Industrial)
Update: Leann Sherman from CT wrote "My Chinese recipes are rather bland, and I read that mushroom sauce is a good substitute for oyster sauce."
Probably a combination of a very rich mushroom sauce, good quality soy sauce and a hint of MSG (if you use it) will be the closest you can get to oyster sauce. Good luck!