Fish / Oyster Sauce
Don't use fish sauce as a substitute.
If you don't have oyster sauce, you can replicate the flavor by combining the following and making a "fake" oyster sauce:
1. Dark soy sauce
2. Brown sugar
3. Rice vinegar
4. Maggi sauce or Worcestershire sauce
Combine and bring to a boil in a saucepan, and then simmer for about 10 minutes at low heat.