In a bowl, combine the sliced chicken with soy sauce, oyster sauce, and sesame oil. Let it marinate for at least 15 minutes to absorb the flavors.
If using dried chow mein noodles, cook according to package instructions until al dente. Drain and set aside. If using fresh noodles, simply toss them with a little oil to prevent sticking.
In a large wok or skillet, heat 1 tablespoon of vegetable oil over high heat. Add the marinated chicken and stir-fry until cooked through and golden brown, about 5-7 minutes. Remove the chicken from the pan and set aside.
In the same pan, add another tablespoon of vegetable oil. Add the sliced onion, bell pepper, garlic, and ginger. Stir-fry for about 2-3 minutes until the vegetables are tender but still crisp.
Add the cooked chicken back into the pan along with the bean sprouts and shredded carrots. Toss everything together for another 2 minutes to heat through.
Gently fold in the cooked chow mein noodles, mixing everything well. Season with salt and pepper to taste. Stir-fry for an additional 2-3 minutes, ensuring everything is well combined and heated through.
Remove from heat and garnish with chopped scallions. Serve hot in bowls.