Oyster sauce and soy sauce are both used to enhance the umami character of savory dishes. Both work well for building savory flavors but use different ingredients to provide their effects. If you are trying to decide between them or want to use one in place of the other, use the SPICEography Showdown below to see how they compare.
How do oyster sauce and soy sauce differ?
Oyster sauce and soy sauce have different ingredients and are made using different methods. Classic oyster sauce is made from an oyster reduction that has been seasoned and thickened; some modern versions are made with oyster extracts. Many of the lower cost oyster sauces that you see on grocery store shelves will contain MSG and other ingredients to give them a stronger umami note.
Oyster Sauce 
Soy Sauce
Oyster sauce may also contain soy sauce for color and flavor. The quality of the sauce depends on the proportion of the oyster reduction to other ingredients like starch, which is sometimes used for thickening. Soy sauce is made from soybeans and roasted wheat that have been fermented with salt.
The two sauces have different consistencies as well. Oyster sauce is always thick either because of starch thickeners or because it has been reduced significantly. While the thickness can vary from ketchup-like to the consistency of maple syrup, it should never be runny. Most versions of soy sauce used in the West are thin, though there are thicker ones. Those are usually given the name soy paste rather than soy sauce.
Oyster sauce is usually dark brown; soy sauce is a much darker brown with some versions being close to black.
If your recipe calls for one, can you use the other?
You can use oyster sauce in place of light soy sauce in most applications and get a similar flavor profile. In fact, it is a particularly good substitute since many oyster sauce producers use soy sauce in their oyster sauce. Oyster sauce will provide saltiness and give foods a strong umami note; however, note the differences in consistency. You may need to thin out oyster sauce to make it work like soy sauce, which may dilute the flavor.
Oyster sauce won’t be a good substitute for dark soy sauce in terms of color. It won’t be able to provide the dark brown color or the strong molasses note.
Light soy sauce can work well as an oyster sauce substitute as long as you bear in mind the thin consistency and dark color. The thinness means that it won’t cling to the exterior surfaces of food as well as oyster sauce will.
Because of its dark color, it will make dishes darker than oyster sauce would. Dark soy sauce will make food even darker and because of its slightly bitter molasses note, it may negatively affect the flavor profile of the dish.
When should you use oyster sauce and when should you use soy sauce?
A thinner oyster sauce is great as a marinade while a thicker one will make a better sauce. Thin oyster sauce is best used for marinades because it is more likely to be absorbed by the meat.
Light soy sauce is best for marinades and dishes that you don’t want to have a very dark color. Use dark oyster sauce to provide color.