Bring a pot of water to a boil and cook the lo mein noodles according to the package instructions.
Drain and toss with 1 tsp sesame oil to prevent sticking. Set aside.
In a small bowl, whisk together soy sauce, oyster sauce, dark soy sauce (if using), brown sugar, rice vinegar, sesame oil, ginger, garlic, and red pepper flakes.
Set aside for later.
Heat 1 tbsp vegetable oil in a large skillet or wok over medium-high heat.
Add the carrots, bell peppers, snap peas, and mushrooms. Stir-fry for 2-3 minutes until slightly softened but still crisp.
Stir in the green onions and cook for another 30 seconds.
Add the cooked noodles to the pan and toss to combine with the vegetables.
Pour the prepared lo mein sauce over the noodles and mix well, ensuring everything is evenly coated.
Cook for 1-2 minutes, letting the flavors absorb.
Remove from heat and garnish with extra green onions or sesame seeds if desired.
Serve as a side dish alongside stir-fries, dumplings, or grilled meats.