Marinate the pork slices with soy sauce, brown sugar, cornstarch, and chili sauce in a bowl for at least 15 minutes.
Cook lo mein noodles according to package directions until al dente. Drain and set aside.
Heat sesame oil in a large wok or skillet over medium-high heat. Add marinated pork and stir-fry until just cooked through, about 3-4 minutes. Remove pork and set aside.
In the same pan, stir-fry the bell pepper, mushrooms, carrots, broccoli, and snow peas until tender-crisp, about 4-5 minutes.
Add water chestnuts to the vegetables and stir-fry for another minute.
Return the cooked pork and noodles to the pan. Add oyster sauce and toss everything together until well combined and heated through, about 2-3 minutes.
Garnish with sliced scallions and sesame seeds before serving.