In a large Dutch oven or saucepan, bring the stock to a simmer over medium heat. Stir in the soy sauce, vinegar, and oyster sauce. Add the mushrooms, reduce the heat to low, and simmer for 4-5 minutes. This will infuse the mushroom flavor into the broth.
Add the ramen noodles and turn the heat up to medium. Cook until the noodles are al dente, stirring periodically, approximately 4 minutes (or according to package instructions). Stir in the sesame oil and scallions. Add salt to taste, if desired.
Many grocery stores now carry ramen noodles without the seasoning packets in the international aisle. Use 6 ounces total in this recipe. They might not be “instant,” so adjust the cooking time according to package instructions after adding the noodles to the liquid. The benefit of using these noodles is that they’re often a better quality product.