What is a good substitute for oyster sauce (for someone with a shellfish allergy)?
The most widely-accepted substitute seems to be various sauces made from mushrooms. I don't want to say "mushroom sauce" because most products don't have that exact name. For example:
Here you have "Vegetarian Mushroom Flavored Stir-Fry Sauce" on the left, and "Premium Shitake Mushroom Flavored Soy Sauce" on the right. I'm actually not familiar with the one in the middle and not sure what the whole label says - it might be the same as the first.
If you go to a specialty Asian food store, you might actually find a product called "vegetarian oyster sauce", which is pretty much the same thing.
Failing that, you can always substitute soy sauce, or hoisin sauce if you want a less watery texture. They won't be exactly the same, but it's the same general idea (dark, fermented, salty sauce rich in glutamates).
"Fish sauce" will be very close to oyster sauce in taste, but may still contain shellfish, so be careful.
Personally, I think there's a pretty big difference between oyster sauce and fish sauce, which tends to be saltier and subtly sweeter.
@logophobe: There is, but certainly less of a difference between oyster sauce and soy sauce, or oyster sauce and almost any other sauce.
Mind that different brands of shiitake soy sauce vary VERY widely in strength.
I had one that I think was the one on the left or perhaps just looked a lot like it and it was disgusting. Also a lot of "vegetarian oyster sauces contain baby prawns, and even fish sauce, so buyer beware and read ingredient. If anybody came here looking for vegan oyster sauce or fish sauce, yes i know that wasn't the question but this answer was nominated by Google as the 'marquee' answer (the boxed one at the top of the page) to "vegan substitute for oyster sauce". Keep Googling for those recipes — not hard to find.
@wide_eyed_pupil labelling such a product vegetarian would, in most localities, be considered in bad faith nowadays.
I'd go with Hoison sauce if I had to. https://en.wikipedia.org/wiki/Hoisin_sauce It's going to be different, but not horribly awful or anything.